+82 



FOSSIL TURTLES OK NORTH AMKRICA. 



Cope did not clearly distinguish this species from P. ademius. One last costal is indeed 

 mentioned as being crost by longitudinal ridges; and he states that if these had been broken 

 up into tubercles he would have referred the specimen to P. ademius. 



In the collection of turtles secured by the American Museum expedition of 1903 there are 



2 specimens which the writer refers to P. molopinus. These were obtained at Grizzly Buttes, 

 from the level known as B. One of these, No. 5941, presents a practically complete carapace 

 and a few fragments of the plastron; the other. No. 5945, consists of a portion of the carapace 

 and some parts of the plastron. These remains show great resemblances to P. ademius, and 

 it is possible that they should be referred there, and that P. molopinus is not different from 

 the former species. There is, however, one difference between the two forms which appears 

 to be constant. This is found in the ornamentation of the outer surfaces. Little if any differ- 

 ence is to be seen in the character of the welts, but the pits, or punctae, of P. molopinus are 

 smaller and closer together than they are in P. eedemius. In the latter there are usually only 



3 or 4 pits in a space of 10 mm.; while in P. molopinus there are quite constantly 6 or 7, 



occasionally but 5. To the eye, the sculpture of the 

 latter has a much finer appearance. In the case of both 

 species, the longitudinal welts are broken up, on the last 

 costals, into elongated tubercles. It is also to be noted 

 that the pits and punctae of P. molopinus are more gen- 

 erally distributed over the carapace than they are in 

 P. eedemius. 



In P. eedemius the neurals, the proximal ends of 

 most of the costals, and the whole of the hindermost cos- 

 tals, are smooth or nearly so, except where varied by 

 the presence of welts. 



The sculpture on No. 5941 is not very distinct, a 

 fact that is due principally to the dirty color of the speci- 

 men. No. 5945 is of a clear sage-green and has been 

 weathered free from all matrix. It shows the welts and 

 the punctae with great distinctness. 



No. 5941 (plate 85, fig. 3; text-fig. 642) has a length 



of 200 mm. and a width of 180 mm. It resembles closely 



the specimen. No. 5978, which has been described here 



as P. eedemius. It was, in life, evidently quite convex. 



It differs from P. eedemius, as represented by No. 5978, 



in having the costals of the sixth pair wholly separated 



by the sixth neural. No. 5945 differs from No. 5978 in 



the same way; but it differs from No. 5941 in having 



a minute seventh neural. This neural is regarded as having no specific value. No. 5945 



further differs from No. 5941 in the somewhat broader neurals, and in having the free 



borders of the costals less abruptly beveled. 



Of both specimens. No. 5941 and No. 5945, the mesial end of the hyoplastron is present. 

 It differs from the same region in P. eedemius in having a notch in the front border, mesiad 

 of the process against which the outer end of the entoplastron rests; but whether or not this 

 might appear with greater age is uncertain. The distal end of the hvpoplastron is present in 

 No. 5945. It differs somewhat from that of P. ademius, but it is impossible to say how con- 

 stant this difference is likely to be. 



The sculpture of the plastron differs from that of the plastron of P. ademius in consisting 

 of smaller and more closely placed pits and punctae. Of course, in making the comparison 

 corresponding regions must be compared. 



Both the specimens here referred to P. molopinus were secured in the Bridger beds of the 

 western end of Grizzly Buttes. Cope figures specimens from the Wasatch of New Mexico 

 under the name P. communis. These are indicated in the text (Wheeler Surv., iv. p. 50) as 

 "var. ii," but in the explanation of plate xxv as "var. i." We must be uncertain about this 

 identification until additional materials of the supposed molopinus have been secured in the 

 Wasatch beds. 



Fig. 642. Plastomenus molopinus. 

 Carapace. Xj. No. 5941 A.M.N. H. 



