M.. I(j00. OSTEOLOGY OF CAilPTOSAVRUS—GILMORE. 225 



only those vertebrap which support sacral ribs, and this interpreta- 

 tion does not include all of those sutured b}^ their centra in adult 

 individuals, as will be discussed later. In the two specimens re- 

 ferred to above, there are preserved 38 and 34 caudals respectively, 

 and I have estimated that probably nine or more would be required 

 to complete the series. In regard to the caudal series in Campto- 

 isairrus, it appears to agree approximately Avith the number found 

 in complete specimens of Iguanodon, which varies from 40 to 48. 



The atlas. — The atlas is composed of four separate pieces, the in- 

 tercentrum, two neural arches or neuracentra, and the odontoid 

 l)rocess. The intercentriun is a subcrescentic block of bone, the longer 

 axis being transverse. Viewed from above, the median surface is 

 concave, forming a hollow in which the anterior rounded portion of 

 the odontoid rests. The anterior part of this concave surface is more 

 deeply excavated, forming a shallow, transverse groove in which a 

 corresponding ridge on the antero-inferior surface of the odontoid 

 fits. On either side of the median depression are the articular faces 

 which look upward and outward, and on which the pedicles of 

 the neuracentra I'est. The posterior view presents a nearly vertical 

 face, rounded only on the median inferior border. Inferiorly and 

 on either side are well developed facets for the articulation of the 

 cervical ribs of the atlas. The anterior face superiorly is deeply ex- 

 cavated, forming the lower portion of the cup for the reception of 

 the occipital condyle, the anterior border being lip-like where it 

 underlaps the articular surface of the condyle. 



Xone of the specimens studied has the upper expanded ends of the 

 neuracentra complete. The articular end is about evenly divided into 

 two faces which meet at an obtuse angle. The posterior face rests 

 upon the intercentrum, while the other looks forward and downward 

 and forms part of the cup for the occipital condyle. Above the 

 articular end just described, the shaft is constricted, but superior to 

 this neck it widens again, but as to its further extent the available 

 material shows this part to be lacking. As in other dinosaurs, the 

 neuracentra articulate posteriorly with the anterior zygapopyses of 

 the axis. 



The odontoid is slightly cupped posteriorly, and though closely 

 applied to the axis, shows no indication of coalescence in any of the 

 specimens studied. The upper surface forms the floor of the neural 

 canal and is slightly concave transversely. The anterior half of the 

 odontoid is slightly constricted, forming a short neck (see 1, fig. 

 11). Inferiorly the surface is rounded, the posterior half having 

 a smooth, transversely rounded articular 'surf ace which is in contact 

 with the upper concave surface of the intercentrum. The smooth 

 anterior end is rounded both vertically and horizontallj^ wdiere it 

 abuts against the posterior end of the occipital condyle. In passing, 

 Proc. N. M. xxxvi— 09 15 



