224 



FOSSIL TURTLES OF NORTH AMERICA. 



\-infml 



infm 2 



^4t- irtf-m 3 



*ft 



-\l-uifm4 



So far as the writer can determine, there is no good reason for separating the fossil genera 

 that have been arranged under the name Adocida: from the living dermatemyds. The name 

 DermatemyJcs was employed by Gray in 1870, apparently a short time before Cope pro- 

 posed the name Adocidce, and ought, under the form Dermatemydida:, to be used for the family. 

 It is difficult to frame a definition of the family. Our knowledge of the few living forms 

 is not complete. Of the great mass of the fossil genera we are acquainted with little more 

 than the shells, and often with only small portions of these. Of no fossil genus have we the 

 skull, except that of Baptemys. The limbs and feet and the caudal vertebrae are unknown. 

 In the shells there is a great variety of structure. Perhaps no other family furnishes so many 

 deviations from what is regarded as the normal condition in turtles. 



Usually in the members of this family some of the hinder neurals are aborted, a condition 

 that permits one or more pairs of the hinder costals to meet their fellows in the midline. In 

 Baptemys, however, there is a full series of neurals. In the living genera, Staurotypus and 

 Claudius, the nuchal bone has long costiform processes, and the possession of these has been 

 regarded as a characteristic of the family; but these processes are absent in Der.natemys ; 

 while in Adocus and Xenochelys, and perhaps others, they are short; in Anosteira they are 

 absent. The plastral lobes are in nearly all cases shortened and often narrowed. In the 



living Dermatemys (fig. 286) the lobes are quite 

 well developt. In nearly all forms the plastron 

 is suturally articulated with the carapace; but 

 in living Claudius the union is ligamentous. 

 In some of the fossil genera the sutures are 

 coarse, forming a transition from the ligamen- 

 tous connection to the closely knit sutures. 

 In Adocus and Baptemys axillary and inguinal 

 buttresses rise to the costals, or nearly so. In 

 probably all genera, except Bastlemys, there 

 is a series of inframarginal scutes crossing the 

 bridge. In Xenochelys and Staurotypus the 

 number is reduced to two. In probably all 

 genera there is some deviation in the arrange- 

 ment of the scutes of theanterior lobe from the 

 arrangement seen in such a primitive turtle as 



Baena. In Adocus and some others, this pro- 



fic. 286. Dermatemys mawi. Carapace and r , , j i r 



plastron. After Boulenger. ceeds no further than the great development of 



the intergulars, by virtue of which the gulars 

 are greatly reduced and pushed far from each 



other. If this should in any cases have proceeded to the entire suppression of the gulars, 

 the usurping intergulars would be mistaken for the abolisht scutes. Undoubtedly, in some 

 cases, there has occurred either a coalescence or a suppression of scutes. In Baptemys 

 wyomingensis there are only 5 pairs on the plastron, excepting the inframarginals. Two pairs 

 occupy the area covered in Adocus by the intergulars, gulars, humerals, and pectorals. 

 The pectorals appear to have advanct so as partly to cover the epiplastrals. Three pairs of 

 scutes ought to be found in front of this; but there is only one pair. In B. tricarinata, there 

 are apparently distinct evidences of a sulcus crossing the front of the entoplastron and the 

 middle of each epiplastron. This probably bounds the humerals in front. It seems probable 

 that the area in front of this sulcus is occupied by the intergulars and that the gulars have 

 been extirpated. 



The plastral scutes of Agomphus appear to be arranged like those of Baptemys. 

 In Xenochelys we find the modifications of the plastral scutes to have gone still further. 

 On each bridge there are only 2 scutes. What we must regard as the intergulars have coalesct 

 into a single scute. This is followed by a scute on each side which does not come to the mid- 

 line. The whole area extending from these to the femoral scutes is occupied by a single pair 

 of scutes. The pair of scutes following the intergular may be gulars, but it appears more 

 probable that the pectorals have advanct still further forward than in Baptemys and have 



