OSTEOLOGY OF CARNIVOROUS DINOSAURS. 



33 



hangs the notch between the prezygapophyses. This projection in the axis of Xo. 

 8367, U.S.X.M., is rugose, .\nterior and posterior horizontal himina from the 

 post and prezygapophj'ses, respectively, meet at the center of the neural arch and 

 continue outward and downward to form the rather weak diapophyses (A. fig. 17), 

 which extends well lielow the level of the neuro-central suture. 



The pleuropojihyses are well-develojjed articular cups on the anterior borders 

 of the centrum and high m position. In this specimen the axis hypocentrum remains 

 firmly jomed to the front of the centrum, but their union remains clearly defined. 

 In aged individuals it doubtless becomes firndy coalesced, as it has in Ceratosaurus 

 (fig. 6, pi. 19). Osborn' finds this element present as a distinct element in Tyran- 

 nosaurus. It is also known to be present in the Sauropod and Predentate groups 

 of dmosauria, but often becomes so completely coalesced as to lose its identity. 



Cll/t 



lit p 



Fig. 17.— Atlas .ind axis of Antrodemus v,vlens Leidy No. 4734. U.S.N.M., \ nat. size. .4, lateral view; B, front 

 view: C, ventral view: a. in, atlas intercentrum: oi. axis; d., di.u'ophysis: in, intercentrum between atlas 

 .\Ni) axis; n, neurapophvsis of atlas; o. odontoid; o. s, sutural surface for odontoid; p, parapophysis; s, neural 



SPINE; Z, -ANTERIOR ZYG.APOPHYSIS; Z* , POSTERIOR ZYG-\POPnY'SIS. 



The h^-pocentrum is a wide block-like bone that joins the anterior end of the 

 axis centrum by a plain articular end. Viewed from the front the upper transverse 

 border with which the odontoid articulates is straight; the lower border is broadly 

 rounded from side to side. The anterior face is concave from above downward and 

 lip-like, where it underlaps the articular end of the atlas intercentrum. The ventral 

 surface is shallowly concave antero-posteriorly and broadly convex transversely. 

 It has a greatest transverse width of 64 mm. and a greatest antcro-postcrior diam- 

 eter of 34 mm. In specimen No. 8367, U.S.N.M., this bone measures 79 mm. in 

 width and 41 mm. antero-posteriorly. 



The axis of Autrodemus may be distinguished from the homologous element 

 in Ceratosaurus by, (1) the absence of a keel on the ventral surface of the centrum; 

 (2) the presence of deep notches between the spine and postzygapophyses ; (3) by 

 the stronger development of the diapophyses. 



1 Bull. Amer. Mus. Nat. Hist., vol. 22, 1906, p. 2S7, fig. 3. 



