MORPHOLOGICAL REVISION 



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Fig. 23. — Inner view of right lower jaw of DiaAictts tp. X %• 

 No. 1076 Univ. of Chicago. 



are as closely united as are the bones of the skull. Just within the low coronoid 

 process there is a large opening on the inner surface of the jaw admitting to a large 

 cavity and a second one on the same side near the anterior end. The outer surface 

 of the jaw is covered with a rough irregular sculpture. 



The lower jaw of specimen No. 1076 University of Chicago (plate 6, fig. 3) 

 differs in some details from that of No. 4684 Amer. Museum. The jaw is singularly 

 testudinate in general appearance. The anterior portion is relatively very high 

 and the coronoid process is inconspic- 

 uous. The anterior portion of the jaw 

 descends slightly to the symphysis, 

 which is narrow, sutural, and formed 

 almost entirely by the dentary. Just 

 anterior to the coronoid there is the 

 enormous posterior opening of the 

 Meckelian cavity, and (a short dis- 

 tance anterior to this) a second open- 

 ing. The narrow bridge separating the 

 two is probably formed by the dentary. 

 The alveolar border is broad and the teeth are placed near the inner edge, being 

 separated from the raised outer edge of the dentary by a shallow groove. It is 

 impossible to determine the outline of the separate elements. 



There are eleven cheek teeth and at the posterior end the alveolus for a small 

 one. Anterior to these is a single incisor and alveoli for four more, making six- 

 teen in all. This is more than in other specimens. 



The vertebral column (plate 4, fig. 3; plates 6, 14). The complete presacral 

 portion of the column in the University of Chicago specimen, No. 1075, has twenty- 

 one vertebrae. The American Museum specimen, No. 4684, lacks the atlas and axis 

 and there is one break in the column where the parts do not fit. As mounted, this 

 specimen has twenty vertebrae. There are two sacrals and a large number of cau- 

 dals, thirty-three in the specimen as mounted, but ten of these are restored in plaster 

 to supply evident gaps. The exact number is uncertain. 



The atlas, determined in part from specimen No. 1075 University of Chicago, 

 has a simple disk-like centrum with the neural arch free and separated into two 

 halves. As described above, the articular face of the occipital condyle is strongly 

 inclined to the axis of the skull, so that there must have been a large preatlantal 

 intercentrum to enable the skull to be held in line with the body. The neural arches 

 strongly resemble those of Dimetrodon; they are semicircular in shape; the inner 

 face is divided by a ridge into an upper half which protected the spinal cord and a 

 lower half which articulated with the centrum. The anterior end of the upper half 

 extended forward so as to touch the surface of the skull just above the foramen mag- 

 num. On the outer surface of each half is still attached the broken anterior zyga- 

 pophysis of the axis. Articular faces for ribs can not be detected. 



The axis has the neural spine only slightly higher than those of the succeed- 

 ing vertebrae, but of very difFerent form. It is thin laterally and of considerable 

 anteroposterior extent. The posterior edge is thicker and nearly vertical, but the 

 anterior edge slopes downward and forward and terminates in a very thin margin, 

 possibly extending between the halves of the neural arch of the atlas. The anterior 

 zygapophyses are small, the posterior full size, and the articular faces of both are hori- 

 zontal. There are well-developed transverse processes. The intercentrum between 

 the axis and the atlas was small; this seems to have been the condition throughout 

 the column with the exception of the preatlantal intercentrum. 

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