OSTEOLOGY OF CARNIVOROUS DINOSAURS. 



33 



hangs the notch between the prezygapophyses. This projection in the axis of No. 

 8367, U.S.N.M., is rugose. Anterior and posterior horizontal lamina from the 

 post and prezygapophyses, 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 below the level of the neuro -central suture. 



The pleuropoplryses are well-developed articular cups on the anterior borders 

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

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

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

 (fig. 6, pi. 19). Osborn 1 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 dinosauria, but often becomes so completely coalesced as to lose its identity. 



am. tn p a-X 



Fig. 17.— Atlas and axis of Antrodemus valens Leidy No. 4734, U.S.N.M., \ nat. size, a, late-ral view; B, front 

 view; C, ventral view; a. in, atlas intercentrum; ax, axis; a"., diapophysis; in, intercentrum between atlas 



AND AXIS; 71, NEURAPOPHYSIS OF ATLAS; 0, ODONTOID; 0. S, SUTURAL SURFACE FOR ODONTOID; p, PARAPOPHY'SIS; S, NEURAL 

 SPINE; 2, ANTERIOR ZYQAPOPHYSIS; Z' , POSTERIOR ZYGAPOPHYSIS. 



The hypocentrum 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 antero-posterior 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. 



i Bull. Amer. Mus. Nat. Hist., vol. 22, 1906, p. 287, fig. 3. 



