by Michael King Macdona
In the 2006 edition of the Hadrianic Society Bulletin, Fleur Kemmers drew attention to numismatic and other evidence suggesting that a chain of auxiliary forts along the Lower Rhine, previously thought to have been established in AD 47, had been constructed in the latter stages of the reign of the emperor Gaius (Kemmers 2006). She postulated that the river Rhine would have been the main supply route for an invasion of Britain and that the most likely purpose of the forts was to protect the supply lines from attack by hostile tribes and possibly to act as transhipment-ports in connection with Caligula’s proposed expedition in AD 40. The dendro-chronological evidence from the one fort that provided it does not entirely support this theory. It indicates that timbers from the earliest wall and ditch were felled in the autumn or winter of AD 40, whereas it is known that the emperor was back in the vicinity of Rome to attend the rites of the Arval Brethren by the beginning of June that year (Smallwood 1984, no.10), having abandoned the enterprise, at least for the time being. We have it from Tacitus (Agricola, 13) that it was well known that Caligula had contemplated invading Britain but that he gave up the idea due to his fickle temperament (ingenio mobili). What this latest evidence does seem to indicate is that this assessment may have been misconceived, that Caligula maintained his intention and was preparing for a later attempt. The construction of a lighthouse (Suetonius, Gaius Caligula, 46), probably at Boulogne (Lindsay 1993, 147-148), may be seen as part of these preparations. As it was, Caligula was assassinated in January 41 but the infrastructure was available to be exploited by Claudius for his invasion in 43.
In considering the seriousness or otherwise of Caligula’s intentions, Dr. Kemmers had occasion to mention one of the most intriguing aspects of the episode, the emperor’s order to his troops to gather sea-shells to be carried to Rome as a symbol of his victory. The sources for this story are Suetonius, Cassius Dio and Aurelius Victor (see Appendix), none of them contemporaries of Caligula, apparently relying upon sources hostile to him. However, Aurelius Victor’s account may be disregarded as an independent authority as it evidently derives from Suetonius. Suetonius places the event immediately after a tendentious account of Caligula’s activities on the German frontier from which Victor infers that the army was expecting to cross into Germany. The description of Caligula assuming variously full armour or the robes of Venus also comes from Suetonius but from a different passage entirely, namely, Calig. 52. Victor’s account is included here simply for the sake of completeness. A brief mention by Orosius of Caligula’s preparations, without referring to the order to collect sea-shells, adds nothing to our understanding of this incident.
Combining the accounts of Suetonius and Dio, the story is briefly told. Having over-wintered in Gaul and engaged in some allegedly farcical operations on the German frontier, Caligula drew up his forces with their equipment on the shores of the Ocean. Briefly putting to sea, he returned to land and mounted a platform from which he ordered the signal for battle to be given. He then commanded the soldiers to gather sea-shells to be carried in his triumph in Rome. Finally, he ordered the construction of a lighthouse and promised the army a donative of one hundred denarii per man. Before departing for Rome, he determined to punish two legions, identifiable as I and XX, ostensibly for their role in the mutiny following the death of Augustus in AD14, although there could have been few, if any, of the men involved in that incident still serving. His first inclination was to slaughter them wholesale but he then decided upon decimation. However, when they showed signs of resistance, he fled. His triumph, at which the triremes involved in the escapade, as well as the shells, were to be displayed, was to be of unprecedented grandeur but ultimately he settled for an ovation, which took place on his birthday, 31st August 40.
Scholarly opinion as to the meaning of all this is, to say the least, divided. It has been argued that the events took place on the Channel coast in connection with the proposed invasion of Britain (Balsdon 1934b, 88-95; Phillips 1970; Flory 1988, 500; Ferrill 1991, 126-129) or on the coast of the Lower Rhine in relation to operations against the Canninefates (Bicknell 1968), or that they did not involve active hostilities at all but were military manoeuvres (Davies 1966), an inspection of equipment (Woods 2000, 84), ceremonies connected with the surrender of the British prince Adminius (Barrett 1989, 137-138; Hurley 1993, 167; Malloch 2001; Wilkinson 2005, 46-48) or even a symbolic battle with the sea (Bicknell 1962). As to the order to collect sea-shells, some commentators have interpreted this literally, while others have regarded it as referring to something other than shells or to have been misunderstood. Thus, in the first case, the shells have been taken to represent the spoils of victory over Oceanus (Barrett 1989, 138), symbols of the surrender of Adminius (Malloch 2001, 555) or connected in some way with the ceremonies surrounding that surrender (Wilkinson 2005, 47), or missiles to be used in the course of training (Davies 1966, 127). In the second, it has been suggested that Caligula was referring to pearls thought to be within the shells (Balsdon 1934a, 18; Flory 1988, 501; Hind 2003), sappers’ huts (musculi) (Balsdon 1934a, 92; Balsdon 1934b, 18) or small British boats (Woods 2000, 83-84). None of these explanations is particularly convincing but possibly the weakest of all is that propounded by G. J. D. Aalders (Caligula, zoon van Germanicus (Assen, 1959), summarized in Wardle 1994, 313, and Woods 2000, 82) that Caligula was using the word conchae in its slang sense of the female genitalia and was dismissing his soldiers to enjoy the services of the local prostitutes.
The weight of the available evidence strongly indicates that there was a serious intention to invade Britain. Suetonius states that Caligula assembled in Gaul legions and auxiliaries drawn from all quarters, held levies with the utmost strictness and collected provisions on an unheard of scale (Calig. 43). Dio alleges that the troops gathered numbered 200,000 or, according to some, 250,000 (Cassius Dio 59, 22, 1). This seems to be an exaggeration of almost Josephan proportions although, if the figures include the grooms, muleteers, sailors and other non-combatants required to transport a large army and to maintain it in the field, they become somewhat more reasonable. Grossly inflated though they still are, it is evident that a large force had been gathered for a major undertaking. It is true that, according to Suetonius, the purpose of the preparations in Gaul was an expedition into Germany but no such expedition took place and all we have is a description of some minor operations across the Rhine. We are told that Caligula had designs on Germany and Britain and that there were rumours that the construction of a massive bridge of boats at Baiae was intended to inspire fear in both (Suetonius, Calig. 19, 3). If a full-scale invasion of Germany were intended, however, there does not appear to have been anything to prevent it and, accordingly, Britain would seem to have been the object of Caligula’s ambitions.
Suetonius’ account of Caligula’s activities on the Rhine (Calig. 45, 1-2) is utterly ludicrous and evidently designed to present him in the worst possible light. The true nature of these operations, however, is betrayed in the author’s Life of Galba (Suetonius, Galba 6, 3). This speaks of a series of measures taken by Galba, as the newly appointed governor of Upper Germany, to toughen up the army of the province, which had apparently become slack under the lax command of his predecessor Gaetulicus. These included rigorous exercises which resulted in his troops, out of the great many assembled “from all provinces”, receiving especial commendation from the emperor. This confirms the presence of a large force and implies that not only Galba’s men but the whole army was engaged in the manoeuvres. It is recommended by Vegetius that new recruits, as well as troops who had not seen action for some time, should be tested in low-level operations before being exposed to full-scale warfare (Epitoma rei militaris 3, 10, 7-11). If (as seems likely) the legions XV and XXII Primigeniae were created by Caligula (Balsdon 1934a, 13-16) to replace experienced legions withdrawn from the frontier to take part in the invasion of Britain, the sorties across the river described by Suetonius, shorn of their farcical elements, would serve this purpose. They would also secure the frontier by intimidating the neighbouring tribes into quiescence while Roman attention was engaged elsewhere. All these activities, it may be inferred, were intended to bring the army to peak efficiency prior to the proposed invasion.
Ancient sources offer little assistance as to why the proposed invasion of Britain came to nothing, despite these extensive preparations and the vast accumulation of men and matériel. Tacitus’ explanation that it was due to Caligula’s unstable personality is unsatisfactory and an additional comment in the same passage that his great efforts against Germany had failed is unsupported by any other evidence. It may be no more than an allusion to the same preparations, mistaking (or misrepresenting) their object. Dio says only that, having reached the coast, he turned back (Dio 59, 21, 3). Various theories have been advanced by modern scholars to fill the vacuum. Dr. Kemmers suggests waning support for Caligula in Rome and in the army (Kemmers 2006, 8, citing A. Winterling, Caligula, eine Biographie (München, 2003)). In Arther Ferrill’s opinion, failure in Germany, the conspiracy of Gaetulicus and Lepidus, and fear of senatorial hostility all played a part but, for him, the main reason was Caligula’s loosening grip on sanity (Ferrill 1991, 129). J.P.V.D. Balsdon, after summarizing earlier suggestions – distrust of the Senate (Willrich), a change in the situation in Britain (Gelzer), and insufficient ships for the crossing (Dessau) – offers his own theory that, as was to occur prior to Claudius’ invasion three years later, the troops mutinied and refused to embark (Balsdon 1934b, 89-91). This idea has been criticized (Malloch 2001, 551) but it is supported by other scholars (Frere 1967, 58; Phillips 1970, 372; Flory 1988, 500; Wardle 1994, 313) and remains the most likely explanation.
The probable reason for such a mutiny is the same as that which would later cause Claudius’ army to rebel, the fear of having to campaign beyond the limits of civilisation (Dio 60, 19, 2). However, other factors may have contributed. It has been suggested that the events on the Channel coast could not have been associated with a proposed invasion as they occurred before Caligula’s departure for Rome, meaning that the invasion would have been attempted too early in the year (Davies 1966, 126; Barrett 1989, 136). It is argued that, as Caligula was outside Rome by the beginning of June and as it would have taken him at least two months to travel there from the supposed embarkation port at Boulogne, he must have begun his journey in the last week of March at the latest. Vegetius declares that from three days before the Ides of November until six days before the Ides of March (i.e. between the 11th November and the 10th March) the seas were closed (Epit. rei milit. 4, 39, 6) and that, even then, navigation remained perilous until the Ides of May (i.e. 15th May) (ibid.4, 39, 9). Merchants might put to sea between March and May in pursuit of commercial gain but an army should be more cautious (ibid.4, 39, 10). Thus, the argument runs, it would have been impractical to have planned an invasion for as early as March. However, this line of reasoning can equally be applied in support of the theory of the invasion having been thwarted by a mutiny. An unexpectedly early assault would have caught the British tribes off-guard and almost certainly ensured an unopposed landing and rapid advance into the interior. Nevertheless, such strategic considerations are unlikely to have meant much to the soldiery who would probably have seen little other than that they were being asked to undertake a hazardous expedition into unknown territory at a time of year when it was considered dangerous even to leave port. With such perceptions, a reluctance to embark is entirely explicable. In addition, Vegetius comments that it was common knowledge that the period around the New Moon is prone to storms and much to be feared by navigators (ibid. 4, 40, 6). In March 40, the New Moon fell on the 1st and 31st of the month1. If the invasion attempt were made towards the end of the month, the soldiers would have had further grounds for apprehension. That fear played a part in the failure of this enterprise finds some support in a speech to her followers that Dio puts into the mouth of Boudicca at the time of her revolt (Dio 62, 4, 1). In this, she reminds them that they had expelled Julius Caesar from the island and had deterred Augustus and Caligula from invasion, making even the attempt to sail there a formidable undertaking2. It is true that there is no indication in the ancient sources of such a mutiny taking place but there are reasons why this might be. There seems to have been a determination to portray Caligula as being mad. This being so, any suggestion that there might be a rational explanation for his actions would be liable to be suppressed. Further, it was known that Claudius’ invasion had almost foundered because his army refused to embark and there would be a natural desire to eliminate any hint that his enterprise could bear a similarity to that of his despised predecessor.
The order to collect sea-shells remains to be considered. Some commentators have touched upon the most likely explanation, the intention on the part of Caligula to humiliate his troops for their mutinous (or cowardly) behaviour (Balsdon 1934b, 92; Bicknell 1968, 505; Phillips 1970, 372-373; Flory 1988, 500; Wardle 1994, 313). However, they either misunderstand the motivation or do not develop the argument as fully as they might. Thus, Balsdon characterizes the command as a quid pro quo for the insult offered to the emperor by refusing to embark, while Phillips suggests it to have been an attempt to bring the soldiers to their senses. Generally, the impression is given that it was a capricious act by an unstable monarch.
In fact, humiliation was, and remained, an established method of punishment in the Roman army, both for individuals and for whole units. It was often inflicted in the field and was sometimes used as a mitigated penalty when more severe action would have been justified. The best known instance of its use occurred after a unit had been decimated. Those not executed were made to camp outside the ramparts and were fed barley rations, rather than wheat. The symbolism is obvious. These men were not fit to enjoy the society of their fellow soldiers or even to eat the same food3. Some humiliation punishments have something of “The Mikado” about them, the punishment fitting the crime or else offering a commentary upon the perceived character of the offender. Thus, Corbulo ordered that a cavalry prefect who had kept his men inadequately equipped with weapons should be stripped and made to stand at his headquarters until released (Frontinus, Strategemata 4, 1, 28). Augustus commanded that centurions who had failed in their duties should stand outside his tent all day without their military belts or holding measuring rods or turves (Suetonius, Divus Augustus 24, 2). The offences are not specified but one may surmise that, in the latter cases, they involved some deficiency in the construction of the camp. Depriving a soldier of his belt denoted that he had behaved in an unsoldierly manner. Again the symbolism is clear. The military belt was the badge of the soldier and he was not fit to wear it.
Aulus Gellius, writing in the second century, states that in former times it had been a military punishment to disgrace a soldier by bleeding him (Noctes Atticae 10, 8). He infers that this had originally been a medical treatment for soldiers suffering from a mental disorder and that it subsequently became a punishment on the grounds that those who offended were not of sound mind. This, he says, applied to many offences but he does not specify which. If cowardice was one of them, the speculation of the French humanist scholar Marc Antoine Muret (1526-1585) that offenders were required to lose with ignominy the blood that they were not prepared to shed for their country (Variae Lectiones 13, p.199, cited in the Loeb edition of the Attic Nights, p.234, n.3) is plausible. According to Frontinus (Strat. 4, 1, 16), quoting Marcus Cato, bleeding at headquarters was a mitigated penalty for theft, in place of amputation of the right hand in the presence of the offender’s comrades. The rationale is not explained but may be that, if a soldier deprived one of his fellows of his property, he should lose something precious to him. Alternatively, the blood so shed may have been regarded as symbolic of that which would have been shed had the full penalty been exacted.
During his withdrawal from Persia in 363, Julian punished the members of a cavalry unit who had fallen back in the face of the enemy by taking away their standards, breaking their lances and forcing them to march with the baggage and the prisoners. He was content to impose a lenient form of punishment in view of the difficulties that lay ahead (Ammianus Marcellinus 25, 1, 7-9). In less pressing circumstances during the advance, he had acted more harshly. When three turmae of cavalry were routed and lost a standard to the enemy, he discharged and put to death ten of those who had fled (Amm. Marc. 24, 3, 1-2)4. Zosimus tells of Julian’s punishment of a 600-strong unit of cavalry that broke during the battle of Strasbourg in 3575. Instead of imposing the penalty prescribed by law, he dressed them in women’s clothing and paraded them through the camp, a punishment (says Zosimus) that he considered worse than death for manly soldiers (Zosimus 3, 3, 4-5).
Perhaps the most bizarre example of humiliation is attributed to the emperor Macrinus (217-218) (Scriptores Historiae Augustae, Opilius Macrinus 12, 4-5). It had been reported to the emperor by one of his frumentarii that two soldiers had had intercourse with the maidservant of their host. The woman was of loose morals but this would have provided no defence; their offence lay not in the fact of the intercourse but in that it constituted an abuse of billeting arrangements. Once guilt was established, the punishment decreed by Macrinus was that two oxen of exceptional size be sliced open and the men thrust inside with only their heads projecting, so that they could talk to each other6.
Other examples could be cited but it is clear that the only limitation on the variety of humiliation punishments would be lack of ingenuity on the part of the commander. It is suggested, therefore, that Caligula’s order to his troops to gather sea-shells is another instance of this type of punishment. It is true that some scholars have argued that to give his men such an order as a means of humiliating them would have been an exceedingly risky thing to do (Barrett 1989, 135 – “a courageous gesture”; Hind 2003, 272 – “extremely dangerous”; Wilkinson 2005, 47 – “playing with fire”) and, possibly, an emperor less convinced of his own divinity would have been more cautious. Caution, however, is not a characteristic to be expected of Caligula.
With these considerations in mind, it is possible to offer a reconstruction of events surrounding Caligula’s abortive invasion of Britain. Admittedly, this involves doing some violence to the chronology of Suetonius and Dio but, as their accounts appear to be distorted already, perhaps the liberty may be forgiven. Having assembled a large force in Gaul and conducted manoeuvres there and on the Rhine frontier, Caligula brought his army to the coast of the Channel at Boulogne. The troops and their equipment were drawn up ready for embarkation and he mounted his tribunal to review them. The signal to embark was sounded but the soldiers refused to comply. Caligula boarded a trireme and sailed a short way out to sea in an attempt to demonstrate that there was nothing to fear and to persuade his men to follow but they still did not move. Returning to land, the exasperated emperor promised them a donative of 100 denarii per man, not an ungenerous amount as is sometimes alleged (Campbell 1984, 189), and urged them to “Go happy; go rich” (abite laeti, abite locupletes). Despite all his efforts, however, they would not sail. In the face of this recalcitrance, Caligula had no alternative but to abandon the expedition. Infuriated, he resolved to punish the two legions, I and XX, at the centre of the revolt. His first thought was to execute every man but the impossibility of this was immediately apparent. The legions are likely to have been brought to full strength, or close to it, in readiness for the invasion; to carry out his wish would require the slaughter of some 10,000 men. Even his second idea, decimation, would have involved killing 1000 and he reluctantly decided against it for fear of armed resistance. Finally, he settled upon humiliation as an appropriate penalty. These men had behaved like frightened children, so he would treat them like children. As children at the seaside might collect sea-shells, they would collect sea-shells. To heighten the humiliation, he would have a triumph in which the shells would be paraded through the streets of Rome as the only spoils that they were capable of bringing back to the Capitol and the Palatine. In the event, the triumph did not proceed. Perhaps the Senate objected that the technical requirements for a triumph had not been fulfilled – for instance, 5000 of the enemy had not been killed in a single engagement (Valerius Maximus 2, 8, 1) – or that to use it merely to punish his troops would demean a solemn ceremony. Perhaps he was persuaded that he risked exposing himself, as well as his soldiers, to ridicule. Whatever the reason, he was later to complain that he had been denied a fairly earned triumph or rather, on this analysis, the opportunity to complete the punishment of his rebellious army.
Readers may like this explanation or they may not; they may prefer one of those previously advanced or none of them; they may adopt Arther Ferrill’s counsel of despair and conclude that rationality should not be sought in the acts of a crazy man (Ferrill 1991, 128). However, if the solution proposed here has merit, it is that it brings an apparently extraordinary action within the established framework of Roman military discipline. It is an extreme example, perhaps, but one that need not be surprising in the case of an emperor to whom extreme behaviour was no novelty.
Suetonius, Gaius Caligula, 46-49
46. Finally, as if he intended to bring the war to an end, he drew up a line of battle on the shore of the Ocean, arranging his ballistas and other artillery; and when no one knew or could imagine what he was going to do, he suddenly bade them gather shells (conchas) and fill their helmets and the folds of their gowns, calling them “spoils from the Ocean, due to the Capitol and Palatine”. As a monument of his victory he erected a lofty tower, from which lights were to shine at night to guide the course of ships, as from the Pharos. Then promising the soldiers a gratuity of a hundred denarii each, as if he had shown unprecedented liberality, he said, “Go your way happy; go your way rich”.
47. Then turning his attention to his triumph . . . He also had the triremes in which he had entered the Ocean carried overland to Rome for the greater part of the way. He wrote besides to his financial agents to prepare for a triumph at the smallest possible cost, but on a grander scale than had ever before been known, since the goods of all were at their disposal.
48. Before leaving the province he formed a design of unspeakable cruelty, that of butchering the legions that had begun the mutiny years before just after the death of Augustus, because they had beleaguered his father Germanicus, their leader, and himself, at the time an infant; and though he was with difficulty turned from this mad purpose, he could by no means be prevented from persisting in his desire to decimate them. Accordingly he summoned them to an assembly without their arms, not even wearing their swords, and surrounded them with armed horsemen. But seeing that some of the legionaries, suspecting his purpose, were stealing off to resume their arms, in case any violence should be offered them, he fled from the assembly and set out for the city in a hurry, turning all his ferocity upon the senate, against which he uttered open threats, in order to divert the gossip about his own dishonour. He complained among other things that he had been cheated of his fairly earned triumph; whereas a short time before he had himself given orders that on pain of death no action should be taken about his honours.
49. . . . Then giving up or postponing his triumph, he entered the city on his birthday in an ovation . . . (trans. J.C. Rolfe, Loeb Classical Library)
Cassius Dio, 59, 21, 3
When he reached his destination, he did no harm to any of the enemy – in fact, as soon as he had proceeded a short distance beyond the Rhine, he returned, and then set out as if to conduct a campaign against Britain, but turned back from the ocean’s edge, showing no little vexation at his lieutenants who won some slight success . . . (trans. E. Cary, Loeb Classical Library)
Cassius Dio, 59, 25, 1-4
And when he reached the ocean, as if he were going to conduct a campaign in Britain, and had drawn up all the soldiers on the beach, he embarked on a trireme, and then, after putting out a little from the land, sailed back again. Next he took his seat on a lofty platform and gave the soldiers the signal as if for battle, bidding the trumpeters urge them on; then of a sudden he ordered them to gather up the shells (κογχύλια). Having secured these spoils (for he needed booty, of course, for his triumphal procession), he became greatly elated, as if he had enslaved the very ocean; and he gave the soldiers many presents. The shells he took back to Rome for the purpose of exhibiting the booty to the people there as well. The senate knew not how it could remain indifferent to these doings, since it learned that he was in an exalted frame of mind, nor yet again how it could praise him. For, if anybody bestows great praise or extraordinary honours for some trivial exploit or none at all, he is suspected of making a hissing and a mockery of the affair. (trans. E. Cary, Loeb Classical Library)
Aurelius Victor, De Caesaribus, 3, 11-12
Similarly he concentrated his legions in one place with the expectation of crossing over into Germany, then ordered them to gather mussels and cockles (conchas umbilicosque) on the shore of the Ocean while he himself went among them at times in the flowing robe of Venus, at other times, in full armour, he would say that he was taking the spoils not from men but from the gods, doubtless because he had heard that according to the Greeks, who love to embellish everything, fish of this kind are (called) Nymph’s eyes. (trans. H. W. Bird, Translated Texts for Historians, Liverpool)
Orosius, Historiarum Adversum Paganos Libri VII, 7, 5, 5
Caligula, with extensive and incredible preparation, setting out to find an enemy for his idle forces, passing through Germany and Gaul, stopped at the Ocean coast within sight of Britain. And when he had received in surrender Minocynobelinus, the son of the king of the Britons, who banished by his father was wandering with a few followers, since grounds for war were lacking, he returned to Rome. (trans. R. J. Deferrari, The Fathers of the Church: A New Translation, Washington D.C.)
1 Eclipse Predictions by Fred Espenak, NASA/GSFC (http://eclipse.gsfc.nasa.gov/ phase/phasecat.html). Website accessed 3rd October 2008.
2 We have it from Dio that Augustus prepared to invade Britain in 34 BC (Dio 49, 38, 2), 27 BC (ibid. 53, 22, 5) and 26 BC (ibid. 53, 25, 2) but was diverted each time for one reason or another.
3 Barley was fodder for horses. One is reminded of Dr Johnson’s definition of “oats” (“A grain, which in England is generally given to horses, but in Scotland supports the people.”), although I sometimes wonder whether this might show more compassion for the plight of the Scottish poor than he is usually given credit for.
4 He may have intended to decimate the offending units, mistaking the requirements of an obsolete punishment. Alternatively, if approximately 100 had fled, it would have been a true decimation.
5 Presumably, the cataphracti equites mentioned by Ammianus (Amm. Marc. 16, 12, 38-41), although there are differences of detail.
6 Given the unreliability of much of the Historia Augusta, I was inclined to regard this story with some scepticism, until I read a newspaper report of the sale at auction of a sketch by Goya entitled “Constable Lampiños stitched into a dead horse” (The Daily Telegraph, July 9 2008, 9). The drawing shows the carcase of a disembowelled horse with the head of the constable, a hated official in mid-18th century Saragossa, protruding from its rump, beset by a pack of dogs. Goya’s handwritten note states that he remained in this position the whole night (Christie’s 2008, 112). The constable’s death at the hands of a group of women of the town, whom he had persecuted, is recorded in another of Goya’s drawings, now in the Metropolitan Museum of Art, New York (Gassier 1973, 459 & 492-493).
J.P.V.D. Balsdon (1934a), ‘Notes concerning the principate of Gaius’, JRS 24, 13- 24.
J.P.V.D. Balsdon (1934b), The Emperor Gaius (Caligula) (Oxford).
A.A. Barrett (1989), Caligula: The Corruption of Power (London).
P.J. Bicknell (1962), ‘Gaius and the sea-shells’, Acta Classica 5, 72-74.
P. Bicknell (1968), ‘The emperor Gaius’ military activities in A.D.40’, Historia 17, 496-505.
J.B. Campbell (1984), The Emperor and the Roman Army, 31 BC-AD 235 (Oxford).
Christie’s (2008), Old Master and 19th Century Drawings, Tuesday 8 July 2008 (London), 110-113 – auction catalogue.
R.W. Davies (1966), ‘The “abortive invasion” of Britain by Gaius’, Historia 15, 124-128.
A. Ferrill (1991), Caligula: Emperor of Rome (London).
M.B. Flory (1988), ‘Pearls for Venus’, Historia 37, 498-504.
S.S. Frere (1967), Britannia: a history of Roman Britain (London).
P. Gassier (1973), The Drawings of Goya: The Complete Albums (London).
J.G.F. Hind (2003), ‘Caligula and the spoils of Ocean: a rush for riches in the far North-west’, Britannia 34, 272-274.
D.W. Hurley (1993), An Historical and Historiographical Commentary on Suetonius’ Life of C. Caligula (Atlanta).
F. Kemmers (2006), ‘Coins, Countermarks and Caligula: The connection between the auxiliary forts in the Lower Rhine delta and the invasion of Britain’, Hadrianic Society Bulletin, n.s. vol. 1, 5-11.
H. Lindsay (1993), Suetonius: Caligula (London).
S.J.V. Malloch (2001), ‘Gaius on the Channel coast’, Classical Quarterly 51, 551-556.
E.J. Phillips (1970), ‘The emperor Gaius’ abortive invasion of Britain’, Historia 19, 369-374.
E.M. Smallwood (1984), Documents Illustrating the Principates of Gaius, Claudius and Nero² (Bristol).
D. Wardle (1994), Suetonius’ Life of Caligula: A Commentary, Collection Latomus 225 (Brussels).
S. Wilkinson (2005), Caligula (London).
D. Woods (2000), ‘Caligula’s seashells’, Greece & Rome 47, 80-87.