288 FOSSIL TURTLES OF NORTH AMERICA. 



1. Shell smooth; no dorsal keel; crushing-surface of upper jaw 



with a feeble ridge; rib-heads well developt Chrysemys 



2. Surface of shell sculptured; rib-heads thread-like Deirochelys 



3. Shell smooth or wrinkled ; crushing-surface of upper and lower 



jaws broad and flat; palatine bones joining vomer in front of 



choanae Graptemy 



f 2 . Crushing-surface of upper jaw with a strong longitudinal ridge. 



4. At least a trace of a dorsal keel; carapace often sculptured; 



crushing-surface of upper jaw with a denticulated ridge highest 



in front Trachemys 



5. Crushing-surface of upper jaw very broad, with a denticulated 



longitudinal ridge; lower surface of lower jaw flat Pseudemys 



i 3 . Plastron completely filling the opening of the carapace; furnisht with both 



lateral and transverse hinges Terra pent 



Genus GYREMYS nov. 



Shell deprest; as broad as long, or broader; with plastron and carapace sculptured as in 

 the Trionychidae; both furnisht with epidermal scutes, the vertebrals very broad. Carapace 

 with a complete set of peripherals. Neurals mostly hexagonal, with the broader end forward. 

 Plastron without fontanels, and joined to the carapace by means of anterior and posterior 

 buttresses. Apparently no mesoplastra present. 



Type: Gyremys spectabdis. 



This genus differs from Glyptops, as represented by G. plicatulus, in the form of the cara- 

 pace, the width being greater than the length, and in the character of the ornamentation of the 

 shell. The lack of mesoplastra, if confirmed, will be decisive in showing its non-relationship 

 to Glyptops. In the latter genus, as well as in Pleurosternon, the suture between the hypoplastra 

 and the xiphiplastra runs across the plastron in a straight line or is even deflected backward at 

 the midline, apparently a primitive character; while in Gyremys the suture is deflected forward 

 at the midline. 



Provisionally this genus is arranged with the Emydidae. No intergulars or inframarginals 

 have been observed, but they may nevertheless have been present. It is possible that the genus 

 belongs among the Dermatemydidae. 



Gyremys spectabilis sp. nov. 

 Plate 44, figs, i, i; text-figs. 357, 358. 



Under this name there is here described a remarkable turtle which was collected in 1876 

 for Professor Cope, by Messrs. Sternberg and Isaac, in the Judith River deposits of Montana. 

 Apparently the remains were never examined by Professor Cope, and the many fragments 

 of which it was composed were put together after it came to the American Museum of Natural 

 History. The specimen bears the number 1 171. 



The bones of this turtle were originally thin; and as a result of the conditions attending 

 fossilization they have been much fractured. Consequently, it is difficult to determine the 

 positions of some of the sutures and scutes, especially those of the carapace. 



As regards the parts remaining, the plastron is complete; the carapace lacks the nuchal 

 bone, the right first peripheral, the left first costal, and several of the left peripherals. 



The shell was a remarkably broad and flat one. The total length of the carapace (plate 44, 

 fig. i; text-fig. 357) was about 440 mm.; the width, 500 mm. Hence, the shell was broader 

 than long. As restored, the height of the carapace above the bottom of the plastron is only 

 70 mm. It was no doubt more elevated during life, but probably not much. 



The ornamentation of the shell consists, where distinctly displayed, of low, short, irregular 

 ridges, separated by grooves of the same width, and occasionally anastomosing. There are 

 about five ridges in 10 mm. The central portions of the carapace, however, are nearly smooth. 

 In most parts of the shell the sculpture is so effaced that its character can hardly be determined, 

 beyond the fact that it resembles that of the Trionychidae. 



However, the relationships of this turtle are not with the Trionychidae, but with the 

 Cryptodira; for the plastron is completely closed and the costals are surrounded by a complete 



