226 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXXVI. 



it may be of interest to note that the atlas of Stegosau7'us is, in 

 nearly all respects, very similar. 



zyg. 



p- T-HQ 



-P 



Measurements of atlas, Speeimen Cat. No. 5.'i73, U.S.N.M. 



mm. 



Greatest length of iuterceutrum antero-posteriorly 23 



Greatest width of intercentrum 58 



The axis. — The axis of specimen Cat. No. 5i73, U.S.N.M., as 

 preserved is fairly complete, and it is upon this that the following 

 detailed description is based, although muny of the facts are sup- 

 plied from the incomplete axes of Cat. Nos. 2210 and 5474, U.S.N.M., 

 and 1877, the holotype of C. dispar, in the Yale University Museum. 

 The centrum is plano-concave, the cup being moderately deep. 

 Medially the centrum is constricted both laterally and inferiorly but 

 without ventral keel. The anterior extremity is more expanded later- 

 ally than the posterior, the width of the former exceeding the total 



leng-th, as shown in fig. 12. On 

 2 the lateral median surfaces are 



two small vascular foramina. 

 These are not present, however, 

 in the axis of Cat. No. 5474, 

 U.S.N.M., but are represented 

 by shallow cavities or depres- 

 sions. 



The neural arch is composed 

 of two parallel plates of bone, 

 which, as they rise from the 

 centrum, gradually converge, 

 uniting above and forming a 

 s h a r p median longitudinal 

 crest. Transversely the neural 

 spine is compressed, but it extends out over the centrum at either 

 end, more especially the posterior (see fig. 11). This portion of the 

 spinous process rises somewhat and flares out into a comparatively 

 thin frill-like plate which overhangs the centrum of the succeeding 

 vertebra. Posterior zygapophyses, which look downward and out- 

 ward, are well developed on the lower borders of the overhanging 

 part. The anterior prolongation of the spinous process is hardly 

 more than an anterior development of the median crest. xVlthougli it 

 appears that the anterior zygapophyses were probably present, this 

 part of the bone in all of the specimens studied is damaged, and their 

 shape and position could not be determined. 



A weak diapophysial process which extends outward and down- 

 ward (see p, fig. 11), is developed on the median, infero-lateral sur- 



o 

 ax. in 



Fig. 11. — (1) Axis and portion op atlas of 

 CAJirTOSAUuus DisPAu Maksh. Cat. No. 

 5473, U.S.N.M., side view, \ nat. size ; 



(2) POSTERIOIt VIEW OF SAME; aX. in, SEC- 

 OND INTEKCENTUUM ; UC, NEURAL CANAL; O, 

 ODONTOID ; p, DIAPOPHYSES ; p. Zyg , POST- 

 ZYG.\POPHYSES ; &, NEURAL SPINE. 



