EMYDID.S:. 353 



The type of the species Emys petrolei Leidy must be regarded as being the two epiplastrals 

 figured by him in 1873. Leidy mentions and briefly describes a nuchal bone and a hypo- 

 plastral, but there is no certainty that these belonged to a member of the same species. The 

 two epiplastrals are the right and the left, but they did not belong to the same individual. 

 Dr. Leidy's figures present a view of the lower surfaces; the figures here presented (plate 46, 

 fig. 3) represent the upper surfaces. 



The individuals to which these epiplastrals belonged appear to have had plastra about 

 200 mm. long, tho perhaps somewhat less. Dr. Leidy thought that the animal belonged 

 to the genus Trachemys and compared it especially with T. scripta (T . scabra). When 

 compared with a specimen of the latter whose plastron is 270 mm. long there appear decided 

 differences. 



The epiplastral lip of T. petrolei was 52 mm. wide; that of the specimen of T. scripta 

 referred to is 60 mm. wide, a difference due to difference of size of the individuals. The lip of 

 T '. petrolei is directed forward; that of T. scripta is rolled downward in front. The thickness 

 of the epiplastrals of T. petrolei at the symphysis is 11.5 mm., somewhat more than that of the 

 specimen of T. scripta, altho the latter is so much larger. The backward extension of the lip, 

 along the midline on the upper surface, is 29 mm. in T. petrolei; in T. scripta, only 23 mm. 

 In the former the thickening referred to ends posteriorly much more abruptly than it does in 

 T. scripta. In both species it may be observed that the gular scutes overlapt the anterior ends 

 of the entoplastron. There can be no doubt that T. petrolei is a species distinct from both T. 

 scripta and T. elegans. The latter species occupies at present the region where T. petrolei 

 was found. T. elegans has a very different epiplastral lip, this being wider; concave above 

 so as to resemble a spout; and not so thick. In both T. scripta and T. elegans the gular sulci 

 on the upper surface of the epiplastrals run outward and forward, while in T. petrolei they run 

 directly forward. 



It will be observed from the figures that there are some differences between the two epi- 

 plastra, but these are not sufficient to indicate distinct species. 



The nuchal bone (plate 46, fig. 4) mentioned by Dr. Leidy has a length of 46 mm. and a 

 maximum breadth of 50 mm. The width at the free border has been 25 mm. The border for 

 articulation with the first peripheral is very concave. Where the sulci bounding the nuchal 

 scute crost the free border there are very deep notches, so that the nuchal scute projects as a 

 prominent tooth. The greatest thickness of the nuchal bone is 10 mm. 



The nuchal scute is 20 mm. long and 7 mm. wide at the base. The width of the scute on 

 the under side of the bone is slightly less. In T. scripta the scute is nearly twice as wide on 

 the lower side as on the upper. In T. elegans the lower side is about one-half wider than the 

 upper. The first vertebral scute of T . petrolei has a width anteriorly of 27 mm., and it has 

 grown wider posteriorly. In the specimen of T. scripta mentioned, the first vertebral is only 

 18 mm. wide anteriorly. In the midline of the fossil nuchal there is a mere suggestion of the 

 presence of a carina; in T. scripta the carina is very distinct. As to the sculpture, there are 

 faint ridges which occupy the same positions as the much more distinct ones in T. scripta and 

 T . elegans. 



The hypoplastral bone mentioned by Dr. Leidy is present, as well as a second belonging 

 to another individual. These are much thicker than the corresponding bones of T. elegans 

 and somewhat thicker than those of T. scripta. Here again we are uncertain whether these 

 bones belong to the same species as the epiplastra. 



The nuchal bone above described resembles that of the species described by Cope under 

 the name Pseudemys bisornatus, here called Trachemys bisornata. We must therefore consider 

 the possibility that Cope's species is identical with that described earlier by Leidy. This 

 question can not be settled until additional materials have been collected at one or both of the 

 localities, Hardin County and Atascosa County. 



Trachemys bisornata (Cope). 

 Plate 56, fig. i. 



Pseudemys bisornatus, COPE, Proc. Amer. Philos. Soc., xvn, 1878, p. 228. 

 Trachemys bisornata, HAY, Bibliog. and Cat. Foss. Vert. N. A., 1902, p. 449. 



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