328 SCOTT. [Vol. XI. 



It is remarkable how many of the approximately contempo- 

 raneous Oligocene genera have reached the same stage in the 

 transformation of the odontoid, and as I have elsewhere shown, 

 this transformation is one of the most convincing cases of par- 

 allelism, it having been accomplished many times independently 

 and always in the same way. Protoceras is in the same stage 

 of the change as Oreodon, Gelociis among the Pecora, Poebro- 

 theriiun among the camels and llamas and MesoJiippjts among 

 the horses. The resemblance is particularly close in the cases 

 of Gelociis and Mesohippiis, while in Poebrothermin the process 

 is wider and shorter, and in Oreodon it is broader and more 

 flattened than in any of the genera named. The significance 

 of this oft repeated change in the character of the odontoid 

 process is to be sought for in the relations between the 

 axis of the skull and the line of the neck. In the short-necked 

 forms with conical odontoid, it will be found that the cranio- 

 facial axis is continuous with the line of the neck, or, at most, 

 that they form a very open angle, while in the long-necked 

 forms with spout-shaped odontoid, the two lines meet at a more 

 or less acute angle. It is, of course, of the highest importance 

 to give the spinal cord a channel without sharp bends, and this 

 is accomplished in long-necked ungulates by making the odon- 

 toid concave. 



In Protoceras the neural spine of the axis is different, not 

 only from that of the Pecora, but also from that of most ungu- 

 lates in general, in forming a great hatchet-shaped plate ; its 

 upper margin descends in a gentle, regular curve, from behind 

 downward and forward, describing an arc of about 45°. The 

 free border is but slightly thickened and rugose and the whole 

 plate is very thin and delicate, though in some cases the hinder 

 border is much thickened. The posterior zygapophyses are 

 prominent, present obliquely downward and outward and have 

 facets which are slightly concave transversely. The transverse 

 processes are short and very slender, and their bases have no 

 great antero-posterior extension along the sides of the centrum. 

 The foramina of the axis are quite different from those found 

 in the Pecora. In the latter the pedicels of the neural arch 

 are perforated for the exit of the second pair of spinal nerves. 



