282 FOSSIL TURTLES OF NORTH AMERICA. 



estimated by Cope to have been about twice the size of the specimens of A. ornata that had 

 then been found. He concluded that it belonged to a distinct species for the reason that the 

 sculpture had not increast in coarseness corresponding to the increase in size of the animal. 

 This, however, is not a sufficient reason. Since the bones increase in size by additions to their 

 borders, the lines and pits of the sculpture once formed will not change in coarseness. A better 

 reason for regarding the bones as those of a species different from A . ornata is to be found in 

 their form. The anterior peripheral, the first of the left side (Cope's fig. 18), has the form of a 

 rhomboid, quite different from that of any anterior peripheral of//, ornata. The lower side of 

 Cope's figure is that of the free border of the bone. The thickness of the bone is 7 mm. The 

 hinder peripheral appears to be the ninth or tenth. It is 7 mm. thick and its free edge is obtuse. 

 In A. ornata, on the contrary, it is acute. 



The length of the anterior peripheral, along the free edge, is 25 mm.; its height is 28 mm. 

 Both these dimensions in the hinder peripheral are 25 mm. The sculpture of the anterior bone 

 is stated to consist of closely packt vermicular ridges which run out flat on the posterior and 

 upper borders. In the posterior the ornamentation consists of closely placed minute tubercles 

 over the whole surface, these being more or less confluent on the posterior and upper borders. 



Mr. R. Lydekker has described and figured a species called Anosteira anglica, from the 

 Lower Oligocene of Hordwell, England (Cat. Foss. Rept., in, p. 143, figs. 34, 35). Dr. G. 

 Baur (Ann. Mag. Nat. Hist., HI, 1889, p. 273) has stated that this can not be distinguish! from 

 Cope's A. radulina. A glance at the figures of the two forms shows that they can not be 

 identical, the sculpture of the English species being far coarser. In A. ornata the suture 

 between the hypoplastron and the xiphiplastron, in crossing from the free border to the mid- 

 line, runs first forward, then backward, then forward again. In the English species it runs 

 obliquely, but directly, inward and forward. It is not probable that two species of the same 

 genus would be so different. 



Genus XENOCHELYS Hay. 



Characters drawn from the type and only known specimen. Neurals 6, the 4 anterior 

 having the narrow end directed forward. Only 7 pairs of costals; those of the sixth and 

 seventh pairs meeting at the midline. Ten pairs of peripheral bones, and 1 1 pairs of marginal 

 scutes; the nuchal scute very small. Plastron joined to the carapace without buttresses. 

 Bridge narrow. Only 5 pairs of plastral scutes. Two inframarginals on each bridge. 



Type: Xenochelys formosa Hay. 



This genus differs from Dermatemys in having no axillary or inguinal buttresses; 5, 

 instead of 6, pairs of plastral scutes; and in the form of the anterior neurals. In the reduced 

 number of peripheral bones and marginal and plastral scutes it resembles Staurotypus. In the 

 latter genus, too, 3 of the anterior neurals have the narrower end directed forward. The 

 width of the bridge of Xenochelys is intermediate between that of Dermatemys and that of 

 Staurotypus. The two genera just mentioned, as well as Claudius, the nearest living relatives 

 of Xenochelys, are inhabitants of Central America. 



Xenochelys formosa Hay. 



Teit-figs. 355, 356. 

 Xenochelys formosa, HAY, Bull. Amer. Mus. Nat. Hist., xxn, 1906, p. 29, figs. 2, 3. 



Of this species there is at present known only the specimen which serves as the type. This 

 was discovered in the year 1904, at Quinn Draw, Washington County, South Dakota, by Mr. 

 Albert Thomson, of the American Museum of Natural History. The deposits in which it was 

 found belong to the lower division of the Titanotherium beds of the White River Oligocene. 

 The specimen consists almost wholly of the shell, and this is somewhat crusht. The plastron 

 is driven upward into the carapace, and a small portion of the left margin of the carapace is 

 wanting. The whole structure of the shell can nevertheless be determined. 



The length of the carapace (fig. 355), measured in a straight line, is 200 mm.; the great- 

 est width 129 mm. The shell is therefore slightly narrower proportionally than that of 

 Dermatemys mawii. The height was moderate. A low rounded carina is found on the nuchal 



