OSTEOLOGY OF THE ARMORED DINOSAURIA. 47 



Axis. — The description to follow is based almost entirely upon the axis of 

 specimen No. 4935. Although present in No. 4934, it is so badly crushed as to be 

 of little value in determining the characteristics of the bone. 



The centrum is plano-concave, constricted medially, with rounded ventral 

 surface without keel. In transverse diameter the enda are expanded, the anterior 

 extremity exceeding the posterior, but unlike the axis of Carnptosaurus the trans- 

 verse diameter of the anterior end is less than the total length of the centrum. 



The neural arch is composed of two thin parallel plates of bone, which curve 

 in above the neural canal, forming a sharp median longitudinal ridge or crest that 

 rises rapidly toward the back. The posterior portion flares out into an expanded 

 ruff-like process which overhangs the centrum of the next vertebra. Well developed 

 posterior zygapophyses are present on the lower borders of the overhanging portion 

 (fig. 14, z'). The anterior zygapophyses are 

 missing from this specimen, but judging from 

 those of the atlas they must have been small 



and placed forward and well up on the side of 5?* 



the arch. J '''■"'■* ■■, ^ x ' 



The articular surfaces for the second cer- ,^t"V- ^ss^^^T 

 vical rib are situated on the anterior superior °~\:' & \\ <i fb^" { 

 part of the centrum and in the center of the 

 neural arch just above the neuro-central su- /°" 

 ture. The diapophyses extend outward and 



J J fj r~ 1 a\ Fig. 14.— Axis and portion of atlas of 



downward (d, fig. 14). _ Stegosaueus sp. Cat.no.4935,U.S.N.M. Side 



The neural canal is exceptionally large, view, j Nat. size, d, diapophtsis; o, odon- 



1 n- .- I • ,v -(1 ,i I • toid; j>, pakapophysis; z', postztgapophtsis. 



subelliptical in outline, with the longer axis '*' 



vertical. Neither of the two axes in the National Museum collections show any 



indication of the presence of a second intercentrum as found ' in Carnptosaurus. 



The odontoid, shown in figure 14, o, is only attached to the axis by matrix, 

 though it may become ankylosed in old individuals, a condition observed by Nopcsa 2 



in Stegosaurus priscus. 



Measurements of axis 3 of No. 4935, U.S.N. M. 



vim. 



Greatest length of centrum 71 



Greatest vridth anterior extremity 64 



Greatest width posterior extremity 54 



Greatest height over all 116 



Cervicals posterior to the axis. — Of the cervical series in specimen No. 4934 the 

 first six vertebrae were found articulated as shown in text figure 16, and in plates 2 

 and 3. Nos. 8, 9, and 10 (see pi. 3), while not connected were so little disarranged 

 that there can be no question but that they represent a continuous series. All of 

 these vertebrae have suffered from crushing, particularly the posterior members of 

 the series, which have the processes so damaged that it is impossible at times to 

 determine their characteristics. 



The cervical centra are all without pneumatic or medullary cavities and the 

 same remark applies to the entire vertebral series. The centra increase in length 



i C. W. Gilmore, Proc. IT. S. Nat. Mus., vol. 32, 1907, p. 164. 



2 F. Baron Nopcsa, Geol. Mag., vol. 8, 1911, pp. 110-111, fig. 1 (a). 



3 Measurements of the axis of No. 4934 will be found in table on page 53. 



