Julian II (AD 360-363)
AE1, Antioch mint, struck ca. AD 361-363
Dia.: 28 mm
Wt.: 8.7 g
Obv.: D N FL CL IVLI-ANVS P F AVG: Diademed, cuirassed bust right.
Rev.: SECVRITAS REIPVB; Bull, head facing, standing right. Two starts above
Ref.: RIC VIII 216, pg 532
From the author 's collection. Ex Frank S. Robinson (Auction 107 lot 389)
Julian was a fascinating historical figure and is one of my favorite emperors of the later empire. He is best known as the last polytheist emperor of the Roman Empire but he was also a talented philosopher, intellectual and military leader. Those interested in reading an outline of his life and rule can do so here.
Julian was a member of the Constantine family and his hard experiences with his Christian relatives almost certainly played a decisive role in his decision to abandon Christianity as a young man. Most of the male members on his side of the family were massacred by his cousin Constantius II after the death of Constantine I in AD 337. Julian only survived this purge because he was a young child at the time. The bloody family dynamics can get rather confusing so I compiled the following family tree to help keep everything straight.
Figure 1 – Constantine Family Tree
When Julian became sole emperor upon the death of Constantius II he is well known to have grown a long beard in imitation of the ancient Greek philosophers and as a physical manifestation of his austerity and philhellenism. This distinguishes his depiction on coins from his immediate predecessors (Constantine and sons) who were all shown as clean shaven.
His beard was apparently a source of ridicule to the largely Christian population of Antioch. Julian wrote an entire satire about his beard that he called Misopogon (Trans: "Beard-hater") which I found to be quite an amusing read. For instance, Julian claims that he was angry at his face for not being "delicate" and "lovely" and so grew;
"… this long beard of mine, to punish it, as it would seem, for this very crime of not being handsome by nature." 
He goes on to contrast his bearded face with the clean shaven Antiochenes and sarcastically pretends to understand the ridicule of the Christians by saying;
"For I myself furnish you with an excuse for it [ridicule] by wearing my chin as goats do… But you, since even in your old age you emulate your own sons and daughters by your soft and delicate way of living, or perhaps by your effeminate dispositions, carefully make your chins smooth, and your manhood you barely reveal and slightly indicate by your foreheads, not by your jaws as I do." 
It is a highly unusual document to say the least. If you want to read the whole essay in its entirety you can do so here: http://www.attalus.org/translate/misopogon.html.
Considering the contemporary uproar surrounding Julian 's beard during
his stay in Antioch I am very pleased to have a coin, struck at Antioch,
with such a detailed depiction of this supposed goat beard to go with
my earlier example of a beardless Julian (before he was openly
Julian II, AD 360-363
AR Siliqua, Lugdunum mint, struck ca. AD 360-361
Wt.: 2.23 g
Dia.: 18 mm
Obv.: FL CL IVLIA-NVS P P AVG, pearl-diademed, draped, and cuirassed bust right
Rev.: VICTORIA DD NN AVG, Victory advancing left, holding wreath and palm frond
Ref.: LVG. RIC VIII 212; Lyon 259; RSC 58†c, IRBCH 1424
From the author 's collection.
The reverse of this coin prominently features a bull standing underneath two stars and the legend SECVRITAS REIPVB (Security of the State).
These bull coins of Julian are particularly famous for one major reason… in a manner that is most unusual for ancient coins it is referenced by the emperor himself. In the above mentioned Misopogon, Julian expresses his displeasure with the people of Antioch because they insulted him "for the devices on his coins ." He frustratingly doesn 't tell us why.
Historians, writers and numismatists both ancient and modern have been proposing meanings for the coin iconography and why it should be so offensive to the people of Antioch ever since.
The Bull References the Golden Calf of the Israelites in the Bible
Figure 2: Right - Moses confronts the Israelites about the Golden Calf (Oil on Canvas, Mid 17th century). Left - Israelites worshiping the Golden Calf (Illustration, 1901).
Surprisingly, this is the oldest theory on record and was proposed in the year of Julian 's death in AD 363 by Ephraem of Nisibis. The story behind the claim is an interesting one. After Julian was killed in his failed invasion of Persia his successor Jovian was forced to surrender Nisibis to save the remnants of the Roman Army. Ephraem tells us that he actually saw the body of the emperor lying in state in front of the city gate as the flag of the Persians waved in the breeze overhead . He was inspired by this event to write his Hymns against Julian in which he mentions the coins.
According to Ephraem the coins are evidence of a conspiracy against Christianity by the Pagans and the Jews and that "on his coins they saw the shameful bull… in that bull they saw their own ancient calf… and they rejoiced that there were restored the calves of Jeroboam ."
The short version of the biblical story is that the Golden Calf was made by the Israelites while Moses was receiving the 10 commandments because they had lost faith. The Golden Calf crops up again in the story of how King Jeroboam tried to prevent the people of the northern Kingdom of Israel from sacrificing in Jerusalem for fear they would rebel and so set up Golden Calves for them to worship in two northern cities. To read more see here.
Considering that Ephraem liked to place all of Julian 's actions in the context of biblical events most historians do not give much credit to his attribution of the type.
The Bull References Julian 's Excessive Sacrifices
According to this theory Julian 's coins reference a sacrificial scene with the bull as the intended sacrifice. The earliest mention of this theory is in the church histories of Socrates and Sozomen (with the later relying heavily on the former 's account). However, these accounts were written as late as 75 years after the death of Julian and make one critical error in support of their view. Socrates claims that the reverse features both a bull and an altar but to date no examples have been found with an altar on the reverse. This leads one to speculation on whether Socrates had actually even seen one of these coins in person or whether we was only aware of them from written sources such as Julian 's Misopogon and made his assertions based on assumption.
However, despite these doubts about the source material this theory seems to me to be entirely plausible.
The Coin Shows the Apis Bull Discovered in AD 362
Figure 3: The Apis Bull
This is perhaps the best known of the theories for Julian 's bull coins precisely because it is such a fascinating possibility. In AD 362 an Apis Bull was discovered in Egypt. The Apis Bull was a sacred animal whose worship in some form stretched all the way back to the first dynasty of Egypt. However, its worship may have lapsed during the Christianization of Egypt under Constantine and his sons. Therefore, the discovery of an Apis Bull and the reconstitution of the cult by Julian seems like an occasion momentous enough to warrant inclusion on the coinage.
However, it should be considered that the bull on the coins does not show the typical iconography of the Apis Bull. In particular it lacks the characteristic sun-disk and uraeus that almost always accompany a depiction of the Apis. It is also worth noting that the mint at Alexandria was one of the only mints not to strike this reverse type which does not bode well for this theory. Despite these misgivings this theory still seems possible.
The Bull Represents the Emperor
This theory is mentioned by J.P.C. Kent in RIC as one of only two theories he deems plausible . The theory is pretty straight forward: Julian is the bull and protects the state. However, in common with all the theories mentioned above this theory doesn 't explain the two stars which, as far as I am aware, show up in that number on coins from all mints that struck this type. Additionally, it doesn 't explain why the Christians of Antioch would take offense to such an innocuous message from Julian which we know that they did. One possibility is that perhaps they willfully misinterpreted the message based on Julian 's paganism.
The Bull Represents Julian 's Astrological Sign (Taurus)
Figure 4: Epic Bull
This is another theory that J.P.C. Kent deems plausible in RIC . In this theory the bull represents the star sign under which Julian was born. This may in part explain the stars as part of the design. However, we do not have any sources that tell us when Julian was born so this theory must remain entirely speculative.
The Bull Represents Mithraism
Figure 5: The Tauroctony Scene from a Mithraic temple found at Fiano Romano, Near Rome (2nd or 3rd century).
Another popular theory is that Julian was an initiate into the Mithraic Mysteries and the reverse is a reference to Mithraic iconography. Central to the iconography of Mithraism was the depiction of Mithras slaying the sacred bull in a cave (tauroctony). This scene usually includes a dog, a serpent and a scorpion along with the bull and there are sometimes two torch bearers that accompany Mithras. In reliefs (see Figure 5), this scene usually includes depictions of Sol (top left) and Luna (top right) along with stars. An astrological element has been proposed as part of the iconography but almost nothing is known about the cult beliefs or rites outside of the artistic remains.
This theory posits that the bull represents the sacred bull and that the two stars represent Sol and Luna, the torch bearers, or stars with some unknown astrological significance. However, it should be noted that a depiction of the Mithraic Bull outside of the tauroctony would be entirely unique and that the scene does not include many of the other elements from the known iconography (such as the snake, dog and scorpion). There is also no evidence that Julian was a Mithraic initiate.
The Bull Represents Helios
Figure 6: A coin of Gallienus showing Sol in connection with a bull. Image courtesy of CNG
This theory suggests that the bull represents Helios / Sol and that the iconography of stars on coins of this period can simply be suggestive of a divine presence in a generic sense. As evidence a link is suggested between the "zoo series" coins of Gallienus (SOLI CONS AVG) where Sol is associated with a bull as protector of the emperor and the coins of Julian where a similar looking bull is depicted as protector of the state (SECVRITAS REI PVB) . Additionally Julian was known to be a great admirer of Helios / Sol and even devoted a hymn to the god entitled Hymn to King Helios.
The best association with Helios and the bull in mythology is recorded in the Odyssey with the story of the Cattle of Helios. When Odysseus and his men stop on the isle of Thinacia, Poseidon sends a storm that keeps them there for a month. When Odysseus leaves to pray to the gods for help his men set upon and eat some of the cattle of Helios which the titan kept on the island. When Helios learns of this and threatens to take the sun to the underworld in his anger Zeus strikes Odysseus 's ship with a lightning bolt and drowns the entire crew except Odysseus.
Figure 7: The companions of Odysseus rob the cattle of Helios (Painting, AD 1554-1556)
It 's an interesting theory but it is not without a few caveats. For one, there is little artistic association between Helios / Sol and a bull outside of the one coin issue from Gallienus shown above. There is also almost a century between that coin issue and when Julian struck his bull coins. Further, while the story from the Odyssey provides a link between Helios and cattle it does not provide a precedent for Helios taking on the form or being represented by a bull himself.
 Griffith, Sidney H., "Ephraem the Syrian 's Hymns 'Against Julian ': Meditations on History and Imperial Power." Vigilae Christianae 41, 1987.
 Socrates, Historia Ecclesiastica; 3.17
 Kent, J.P.C., The Roman Imperial Coinage, Vol. VIII, The Family of Constantine; London, 1981.
 Woods, D.; Julian Gallienus and the Solar Bull; ANJ Volume 12, 2000; pp. 157-169