No. 2.] THE RELATIONS OF PROTOCERAS. 2>'^'J 



of which the spine rises. As usual in the Pecora, the arch 

 is perforated by foramina for the first pair of spinal nerves. 

 The posterior cotyles for the centrum of the axis are more 

 oblique and less directly transverse than in the Pecora ; they 

 are quite high, but of no great width, and the articular sur- 

 face is reflected within the ring of the atlas, where it forms 

 a broad, continuous facet for the odontoid process. There is, 

 however, no facet upon the hinder face of the inferior arch for 

 the centrum of the axis below the odontoid, such as occurs in 

 the Pecora. The transverse processes have no great width, 

 but they extend out more widely than in Cervits or in Moschus, 

 especially toward the posterior end ; they are pierced by foram- 

 ina for the inferior branches of the first pair of spinal nerves, 

 but there is no vertebrarterial canal, as there is in AgriochceruSf 

 the swine, etc. 



The axis (PI. XXI, Fig. 7) is peculiar in many ways. The 

 centrum is very long, much more so than in any other vertebra 

 of the neck, and is broad and much depressed in front, con- 

 tracted in the middle, and subcylindrical behind. The poste- 

 rior surface is obliquely placed and deeply concave, forming a 

 hemispherical pit for the centrum of the succeeding vertebra. 

 The ventral surface of the entire centrum is marked by a 

 prominent but very thin hypapophysial keel. The articular 

 surfaces for the atlas are narrow and high, though extending 

 less upward along the sides of the neural canal than in the 

 Pecora. In the latter the two surfaces are fused together 

 beneath the odontoid process, forming an uninterrupted facet, 

 and their dorsal ends pass into the pedicels of the neural arch. 

 In Protoceras the articular surfaces do not extend beneath the 

 odontoid, but are completely separated by a wide and deep 

 emargination, and a still more extensive notch divides the 

 dorsal end of each surface from the neural arch. The odon- 

 toid process is neither conical nor spout-shaped, but rather 

 semicylindrical, with bluntly rounded point. The upper sur- 

 face bears a median ridge with a shallow fossa on each side of 

 it, and the ventral surface is gently convex from side to side. 

 This shape of odontoid is the half-way stage in the conversion 

 of the conical into the spout-like form. 



