508 



FOSSIL TURTLES OF NORTH AMERICA. 



Fig. 667. Aspiderctcs granger:. 

 Carapace of type. X 5. 



This species needs to be compared with Aspideretes ellipticus and A. guttatus. The form of 

 the carapace is about the same in the three. It is broader than in the typeof/f. ellipticus, but 



slightly narrower than in another specimen. The pre- 

 neural of the species last named is, in all the known 

 specimens, considerably longer and broader than in the 

 type of A. granger^ while the neurals are broader. The 

 proximal ends of the costals of the first pair are narrower 

 in A. grangeri than in A. ellipticus, their width being 

 contained in the length of the carapace 6.33 times in the 

 former and only 5.5 times in the latter. The nuchal is 

 longer, its length being contained in the length ot the 

 carapace two times; in A . ellipticus two and a half times. 

 The sculpture of the carapace is coarser in A . ellipticus 

 than in A. grangeri, there being in the former 4 or 5 

 pits in a line 20 mm. long while in the latter there are 

 from 5 to 7. In making comparisons between the pits of 

 the two species, measurements must be taken on corre- 

 sponding parts of the shell; 36 measurements taken on 

 all parts of the carapace of 2 specimens of A . ellipticus 

 show that there are, on an average, 4.44 pits in a 20 mm. 

 line; 18 measurements made on corresponding parts of 

 the carapace of A. grangeri indicate an average of 5.83 

 pits in the distance named; that is, the pits of the former 

 species have an average diameter of 4.5 mm. while those 

 of the latter have an average diameter of 3.43 mm., a 

 difference of a little more than 1 mm. In A. ellipticus 

 the small pits are found almost wholly on the nuchal and close to the border of the shell toward 

 the front. The writer regards the finer sculpture of the specimen here described as an indi- 

 cation of a distinct species. 



The sculpture of the carapace of this supposed species is apparently identical with that 

 of A. guttatus and it is possible that the specimen ought to be referred to the latter species. 

 However, the type of A. grangeri has a well-developt preneural bone and no fontanel on each 

 side of it; whereas, the specimen of A. guttatus has large fontanels and only a rudimentary 

 preneural. A . grangeri is a strongly archt species. It is possible that the flatness ot all the 

 specimens of A . guttatus is due to distortion during fossihzation. 



Genus AXESTEMYS Hay. 



Axestus, Cope, Proc. Amer. Philos. Soc, xn, 1872, p. 462. (Preoccupied in entomology.) 

 Axestemys, Hay, Amer. Geologist, xxiv, 1899, p. 348. 



Plastron, so far as known, without the usual pits and ridges. Nuchal bone partly separated 

 from the first costals by fontanels. Probably no preneural. 

 Type: Axestus byssinus Cope. 



Professor Cope based the genus Axestus on the species A. byssinus, which was represented 

 by the left xiphiplastron, most of the pelvis, the larger portions of both femora, both humeri 

 with distal ends missing, the right scapula and coracoid complete, a cervical vertebra, two 

 sacrals, a few caudals, and some phalanges. This type belongs to the U. S. National Museum, 

 but the caudal vertebrae and the phalanges appear to have been lost. 



The character on which Cope relied for separating this genus from other genera ot Trio- 

 nychidae is found in the absence of superficial sculpture on the lower side of the plastron, and 

 the presence there of a layer which "is marked with decussating lines of osseous deposits, as in 

 woven linen." The other characters given by him in his generic descriptions are probably 

 common to most of the Trionychidae. 



This layer of decussating bony fibers (plate 104, fig. 4) is also common to various fossil 

 and living species of Trionychidae, and may be seen in both the carapace and the plastron of 

 Platypeltis spimfera near tin- borders of the bones where the sculptured layer thins out. 

 As Cope says, such a layer is found in the dermal scutes of the Crocodilia; and Marsh has 



