546 



FOSSIL TURTLES OF NORTH AMERICA. 



have been quite vestigial. The costals of the first pair are, at their proximal ends, much 

 wider fore and aft than those behind them, being about 53 mm. wide, and they increase in width 

 toward their distal ends. The third and fourth costals are narrow at their proximal ends. 



being respectively 36 mm. and 33 mm. wide, and they 

 expand little toward the distal ends. The fifth and sixth 

 are likewise narrow proximally, but they expand rapidly 

 toward the margins of the carapace. The seventh cos- 

 tals extend on each side about 66 mm. from the midline. 

 The free ends of the costal ribs extend little, it any, 

 beyond the margins of the carapace, thus affording evi- 

 dence that the individual was an aged one. 



The carapace is thick and solid. Near the proximal 

 ends the costals are 6.5 mm. thick and this thickness 

 increases toward the margins of the carapace, where it 

 amounts to 8 mm. in front and behind, and to 12 mm. 

 and even 15 mm. at the sides. The borders of the cara- 

 pace are not beveled off as in manv species. Anteriorly 

 and posteriorly the edge is rounded; at the sides the 

 upper layer of bone overhangs the lower, and a groove 

 runs alonw between them. 



There was probably no preneural. The first neural 

 is missing, but the proximal border of the left first costal 

 indicates that the neural was long and narrow, with the 

 narrower end forward. Only a fragment of the second 

 neural remains. The third neural had a maximum width 

 of about 23 mm.; the fourth a width of 20 mm.; the fifth 

 a width of 17 mm. The more posterior neurals have not 

 been preserved. 



The nuchal bone is relatively small. Its lateral extent is 158 mm.; its fore-and-aft measure- 

 ment, 29 mm. The outline of the anterior border is nearly straight, and the immediate edge 

 is rounded and about 7 mm. thick. 



The sculpture of P. externa resembles closely that of P. heteroglypta, consisting of large 

 pits, about two in a line 10 mm. long, on the neurals and the proximal ends of the costals, and 

 of smaller cells, three in 10 mm., on the distal ends of the costals. The large pits have broad, 

 low, and rounded walls surrounding them, while the walls of the smaller pits are more sharply 

 defined. On the outer ends of the costals the pits show some tendency to arrange themselves 

 in rows parallel with the margins of the carapace. 



This species differs from P. heteroglypta in its greater breadth as compared with the length, 

 and in the thickened and grooved lateral borders of the carapace. 



The left hyoplastron and hypoplastron are complete, except the tips of the outer processes. 

 The two bones are thoroly co-ossified. The sculptured layer, the callosities, extends to 

 the midline. The length of the hyohypoplastral suture is 176 mm. The width of the bridge 

 is 62 mm. Here the thickness of the hypoplastron is 1 1 mm. The hyoplastron is somewhat 

 thinner. Near the inner end of the anterior border of the latter bone there is a deep notch for 

 the entoplastron. At the hinder outer angle of the hypoplastron is a notch for a process of the 

 xiphiplastron. The humerus differs only in details from that of P. spun fern. Its total length 



Fig. 



. flatypeltts externa. Cara- 

 pace of type. X 4. 



is 87 mm. 



Platypeltis leucopotamica (Cope). 

 Plate 113, figs. 1-3; text-fig. 704. 



Trionyx leucopot amicus, Cope, Contrib. Canad. Palasont., ill (4 to), 1891, p. 5, plate i, figs. 8, 9. Ami, 



Science (1), xvm, 1891, p. 53. Hay, Bibliog. and Cat. Foss. Vert. N. A., 1902, p. 454. 

 Trionyx punctiger, Cope, Contrib. Canad. Palaeont., in (4 to), 1891, p. 5. Hay. loc. cit., p. 454. 



The type of this species is a fragment of a costal plate ^^ mm. long and 25 mm. wide, 

 which was found in the White River deposits of the Cypress Hills, District of Alberta, British 

 America. Professor Cope's description was reinforced by some fragments of what he regarded 

 as the same species, which he had obtained in the White Buttes of North Dakota. One of 



