506 



FOSSIL TURTLES OF NORTH AMERICA. 



in the American Museum of Natural History is 5930. The convexity and the outline of the 

 carapace are very similar to those of the type. The front margin is somewhat more excavated 

 than in the type, and the lateral borders are more smuated. This is caused by the extension 

 of the sculptured layer out on the distal ends of the ribs. 



The length of the specimen is 450 mm.; the greatest width, 380 mm. The preneural is 

 relatively broader anteriorly than in the type, and the sixth neural is not developt. In the 

 latter respect we may find variations in our living species of Trionychidae. 



With this specimen from Wyoming comes a considerable portion of the plastron. This 

 includes the median half of the left and a fragment of the right hyoplastron, and a considerable 

 part of the right hypoplastron. These bones are thick and heavy, the thickness of the bones 

 along the suture joining them on the narrowest part of the bridge being 16 mm. From this 

 point the thickness is reduced each way to about 10 mm. at the median and outer borders of 

 the bones. At the narrowest part of the bridge the width of the hyoplastron is 50 mm.; of the 

 hypoplastron, 42 mm. The xiphiplastron has joined the hypoplastron by the usual two 

 digitations. The lower surfaces of the hyoplastra and the hypoplastra are occupied by a 



655 



665. 



FIGS. 665 AND 666. Aspideretes ellipticus. 

 665. Carapace of type. XJ. 666. Plastron. Xj. 



nearly obsolete sculpture of pits and ridges. This is most distinct on the outer ends of the 

 bones, where we find 3 pits in a line of 10 mm. This sculpture and the bone supporting it 

 have approacht nearly to the midline, an indication that the individual was an old one. 

 The hyoplastra and hypoplastra resembled closely those of Platypeltis ferox, of Florida. 

 Text-fig. 666 represents what is known of the plastron of this species. The figure is con- 

 structed partly from the specimen just described, partly from the following one. 



A still larger example of this species was collected during the summer of 1903 near the 

 mouth of Cottonwood Creek, Wyoming. Its catalog number in the American Museum of 

 Natural History is 5979. The bones of the median line and of the left side are complete, 

 except that the third neural is wanting and portions of the fifth, sixth, and seventh costals. 

 Some parts of the right side are present. The right hyoplastron is quite complete. 



The length of this specimen is 510 mm.; the width was close to 445 mm. It displays a 

 greater convexity of the carapace than is seen in the other specimens. The nuchal and the 

 preneural resemble closely those of the type. The sixth neural is present, but small. Altho 

 the individual was so large and evidently aged, the free ends of the ribs project beyond the 

 margin of the carapace a distance of about 40 mm. The hyoplastron is identical with that of 



