Chapter II. 



A DESCRIPTION OF ASPIDOSAURUS NOVOMEXICANUS WILLISTON. 



By E. C. Case and S. W. Williston. 



Very few fossils were found on the west side of the Puerco River opposite 

 El Rito, but from a small lens of clay inclosed in a bed of sandstone Mr. Paul 

 Miller collected several associated bones which prove upon preparation to be the 

 skull, portions of the vertebral column and carapace, the scapulae, femora, humeri, 

 portions of the pelvis, and some parts of the smaller Hmb bones of an animal 

 indistinguishable from Aspidosaurus novomexicanus Williston. The type of this 

 species (No. 8io Yale University Museum) is only partially preserved and the 

 identification of the present specimen may not be accurate, but the resemblance 

 of the parts preserved, especially the skull, is so close that it seems best to use the 

 earlier name, provisionally, until more material can be obtained. 



The most striking thing about this fossil is the strong resemblance of the 

 skull to that of Cacops aspidephorous Williston from Texas. Aside from the condi- 

 tion of the otic region, the two skulls are almost identical. The same can be said 

 for the pelvis, the limb bones, and the vertebral column, aside from the dorsal 

 armor; this last is apparently composed of a single row of plates formed by expan- 

 sions of the neural spines, the typical condition for the family Aspidosauridm. 

 A fragment of detached armor accompanying the specimen seems to be composed 

 of two plates to each vertebra, but as the fragment consists of the plates alone, 

 this point is not absolutely certain. The plates do not lie in two vertical series, 

 as in the Dissorophidce, but on the same level. On two of a series of separate 

 plates, each being one of a pair, there is a descending process evidently for attach- 

 ment to a neural spine; the presence or absence of a descending process in the 

 others can not be made out because of the condition of the specimen. The mean- 

 ing of this peculiar arrangement is not evident. Perhaps, as suggested by Willis- 

 ton, the forms included in the families AspidosauridcB and DissorophidcB are more 

 closely related than has been supposed and this form may be a connecting link. 

 However, the separation of the two families is very definite and easily determined 

 and it seems desirable to wait for more evidence before abandoning the distinction. 

 It is possible that the plates may come from the caudal region or (less probable 

 but still possible) they may belong to another animal; this last suggestion is 

 strengthened by the extremely slender descending process, which is far too deli- 

 cate to have been a part of any of the neural spines preserved. 



The skull: There is a strong resemblance between this skull and that of Cacops, 

 and this extends to all minor details if we except the condition of the otic region 

 and the ridges which run posteriorly from the orbit. In Cacops, WiUiston described 

 two ridges running back from the posterior inner comer of the orbit, inclosing a 

 triangular space; in this specimen there is but one ridge, the upper. The pos- 

 terior portion of the skull has been nearly perfectly preserved on the left side, but 

 that of the right has been largely restored in plaster. Fortunately the ends of the 

 tabulare bones and the quadrates are preserved on both sides and this renders it 

 very certain that in this specimen the otic opening was a deep notch, not closed 

 by a posterior bar of bone as in Cacops and Dissorophus. This determines the 



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