TRIONYCHIDiE. 5O9 



described and figured the dermal scutes of a dinosaur, Nodosaurus textilis, which are markt 

 by similar, but coarser, interwoven bony fibers. 



However, since there appears to be an unusual abundance of this layer of fibers and a 

 nearly complete absence of the sculptured layer, the so-called callosities, it appears advisable 

 for the present, to retain the species in a distinct genus. There are reasons for believing that 

 the nuchal bone also will offer some characters when it is completely known. 



Axestemys byssina (Cope). 



Plate 104. fig. 4; text-figs. 668, 669. 



Axestus byssinus, Cope, Proc. Amer. l'hilos. Soc, xn, 1872, p. 462 (A. bysimus, misprint); U. S. Geol. 



Surv.. 6th Ann. Rep., 1X72 (1873), p. 616; Vert. Tert. Form. West, 1884, p. 116, plate xv, figs. 1-12. 



Axestemys byssinus, Hay, Bibliog. and Cat. Foss. Vert. N. A., 1902, p. 455; Amer. Geologist, xxxv, 1905, 



P- 337- 



The type of this species, now in the U. S. National Museum at Washington, has been 

 studied hv the present writer. The type was found on Black's Fork of Green River; the second 

 specimen on upper Green River. Both probably belong to level B. These bones were figured 

 bv Cope in his great work The Vertebrata of the Tertiary Formations of the West, the 

 figures having half the size of nature. These bones are from 2.33 to 3 times as large as the cor- 

 responding ones of a specimen of Platypeltis whose carapace is 147 mm. long. The animal 

 therefore had a carapace probably over 420 mm. in length. 



The xiphiplastron (plate 104, fig. 4; text-fig. 668) differs from that of Platypeltis spinifera 

 in having the posterior process narrower and more elongated and in having the callosity wholly 

 devoid of the layer which ordinarily furnishes the sculpture, but composed of a layer of 

 textile-like fibers of bony tissue. The thickness of the central portions of this bone is 10 mm., 

 that of the living species just referred to being about 3 mm. Hence the thickness of the 

 plastron of the fossil species under consideration is proportionally as great. The greatest 

 length of the fragment of xiphiplastron is 166 mm. It probably had a length originally of 

 188 mm. The shoulder-girdle seems not to have differed in any important respect from that of 

 Platypeltis spinifera. The humerus, so far as represented, is like that of the living species 

 referred to, except that the head appears to have been more narrowly oval, with the plane 

 thru the long axis parallel with the long axis of the shaft of the bone. 



The femora are like those of Platypeltis spinifera, except that there is a very distinct ridge 

 on the middle of the upper surface of the distal half of the shaft. The pelvis appears not to 

 have offered anv difference worthy of mention. The cervical vertebra which has been preserved 

 is certainly the seventh, agreeing in even way with that of Platypeltis spinifera. Its centrum 

 is just 3 times as long as that of the corresponding cervical of the specimen of Platypeltis 

 referred to above. 



Professor Cope mentioned another bone, a hypoplastral, which, on account of its great 

 thickness, he thought might belong to a distinct species. This bone is now in the possession 

 of the American Museum of Natural History and bears the number 1034. Its greatest thick- 

 ness, about the middle of the bone, is 14 mm. and this is not so much greater than that of the 

 xiphiplastron that it can not belong to the same species. The lower surface is like that of the 

 xiphiplastron of the type. The form of this bone does not differ from that of the same bone of 

 Plat vpeltis. 



The American Museum also possesses a number of bones which include the greater 

 portion of the left half of the nuchal, most of the left first costal, fragments of several other 

 costals, and a fragment of plastral bone. They bear the Museum's number 1046. The 

 nuchal bone shows conclusively that the species is not T. scutumantiquum, which is suggested 

 hv the character of the sculpture; while the remains of the plastron prove that they belong to 

 Axestemys. The latter are thick, and hence are undoubtedly of the same species as the thick 

 hvpoplastral mentioned In Cope. Until additional materials are forthcoming we may refer 

 them to A. byssina. The piece of the plas.tron of No. 1046 is about 8 mm. thick, near the base 

 of one of the processes directed toward the midline, and the lower surface has the textile-like 

 surface of the type specimen of the species. The portion of the nuchal (fig. 669) remaining has a 

 fore-and-aft extent of 5^ mm. and a total lateral extent of 102 mm. The mesial end approaches 



