0. P. Hay — Fossil Turtles in the Marsh Collection. 275 



neural, the type of the proposed genus being Trionyx gangeti- 

 cus Cuvier. The first neural of A. beecheri is hexagonal, with 

 the narrow end directed forward. There is present a second 

 neural of similar form and a portion of a third. The remain- 

 ing neurals are missing. In all probability there were eight 

 pairs of costal plates, but the eighth is represented in this 

 specimen by the free portion only of the corresponding rib. 



The sculpture of the carapace consists of a network of ridges 

 enclosing rather deep pits. Usually these pits are without 

 definite arrangement, but on the distal ends of the costals they 

 arrange themselves in rows parallel with the free borders of 

 the carapace. The walls surrounding the pits rise abruptly 

 from the bottoms of the. latter; whereas, in A. foveatus, the 

 walls slope upward gradually from the centers of the^ pits. 

 Furthermore, in the latter species, the pits on the proximal 

 ends of the costals are likely to have quite wide flat spaces 

 between them. 



The plastron is complete. The entoplastron is truncated in 

 front, with a slight notch at the midline. The branches 

 include between them less than a right angle. The epiplastra 

 are broad at their anterior ends. They resemble greatly the 

 same bones in Aspidonectes mutious. The hyoplastra are not 

 coossified with the hypoplastra. Between the inner ends of the 

 hyoplastra is a large fontanelle which is bounded in front by 

 the entoplastron. Between the inner ends of the hypoplastra 

 is another fontanelle which extends backward to the xiphi- 

 plastra. There is no fontanelle between the latter bones. The 

 bridge, where narrowest, has a width of 64 mm . 



The whole lower surface of the hyoplastra, the hypoplastra, 

 and the xiphiplastra, is covered with a sculpture like that of 

 the carapace, except that it is finer. 



The cervical vertebra seen in Plate XVI is probably the fifth. 

 Its length is about 60 mm . Seven caudal vertebrae are preserved, 

 forming a series 122 mm long ; but there were others which have 

 been destroyed. They are very similar to those of Aspido- 

 nectes spiniferus. The shoulder girdle, the fore limbs, the 

 pelvis, and the hind limbs present no important differences 

 when compared with those of modern Trionychidse. 



It appears probable that this individual was a female of 

 mature age. 



The U. S. National Museum possesses a Trionychid which 

 was collected by one of Professor Marsh's parties, while he was 

 vertebrate paleontologist to the U. S. Geological Survey. It 

 was obtained in Converse County, Wyoming, and is referred to 

 A. heecheri. It shows the carapace to have been high and con- 

 vex ; also, that it had a preneural, six neurals, and eight pairs 

 of costal plates. 



