TRIONYCHIDj'E. 



527 



The pits of the plastral hones are smaller than those of the carapace, about 7 in a 20 mm. 

 line. Toward the mesial border of the bones the pits become obsolete and there comes into 

 view a meshwork of bony fibers such as Cope found in his Axestus byssinus (Axestemys 

 byssina). There is, however, no reason for thinking that the present species is identical with 

 Cope's species just mentioned. 



The cervical vertebra present appears to be the sixth. It has a length of 58 mm. An esti- 

 mate shows that the length of the neck was about 380 mm. The bone appears to be slenderer 

 than the corresponding bone of a specimen of Plat y pelt is spinifera. 



The fragmentary bones belonging to the shoulder and pelvic girdles and to the limbs 

 furnish few noteworthy data. The total length of the humerus is 125 mm. The shaft is 

 slenderer than that of Axestemys byssina; the least diameter of the former being 8 mm., that 

 of the latter 17 mm., while the length of the humerus of Axestemys appears not to have been 

 much greater than that of the present species. 



This species is at once distinguish! from A . salebrosa by the greater development of the 

 costals of the two hinder pairs and by the greater width of most of the neurals. The greater 

 breadth of the carapace, the greater length of the nuchal from side to side, and the longer 

 eighth costals distinguish it from A. egregia. Comparison with A. concentnca is required. 



Amyda mira sp. nov. 

 Plate 105, fig. i; plate 106; text-figs. 684-686. 



Under the above name is described a trionychid which was discovered by the American 

 Museum party of 1905, at Grizzly Buttes, Wyoming, in level B, of the Bridger Eocene. The 

 specimen bears the number 6130. It furnishes the skull, without the lower jaw; the carapace 

 complete; and the plastron, excepting the entoplastron, the epiplastra, and a portion of the 

 left hyoplastron. 



The carapace includes 8 pairs of costal plates and there is no preneural; hence the genus 

 Amyda is indicated. The carapace (plate 105, fig. i) is about as wide as long, broadly 

 rounded behind, more sharply rounded in front, and of considerable convexity. The length 



in a straight line is 263 mm.; the extreme width is 252 mm.; 

 the height of the center is 62 mm. above the borders. On the 

 sides the free borders of the costals are beveled off; in front, 

 the nuchal and the first costal are trimmed off at right angles 

 with the upper surfaces. 



The nuchal is 27 mm. wide, 134 mm. from extremity to 

 extremity, and 97 mm. from its union on the free border with 

 the second costal to the same point on the other side. The 

 extremities of this bone are contained each between two plates 

 of the second costal. 



There are 7 neurals, all except the first, the sixth, and the seventh being hexagonal, with 

 the broad end behind. The dimensions are given in the table. 



Of the costals the sixth and the seventh meet their fellows at the midline. Those of the 

 seventh pair are well developt. They join along the midline a distance of 30 mm. and each has 

 a width along the free border of about 33 mm. The extremities of the various costals have a 

 thickness of 5 mm. at the sutural edges and of about 8 mm. thru the rib. The ribs are markt 

 off distinctly the full length of each costal plate. Those along the sides of the carapace project 

 about 20 mm. beyond the border of the disk, but that of the eighth costal projected about 

 40 mm. 



The surface of the carapace is ornamented with pits and grooves and intervening ridges. 

 On the rear are 10 or 12 distinct longitudinal welts. The ridges are narrow, sometimes sharp. 

 The pits frequently coalesce, especially on the distal ends of the costals, to form grooves. These 

 show a tendency to run parallel with the axis of the animal. On the neurals the pits are smaller 

 than on the costals, about 6 of them in a line 15 mm. long. On the proximal ends of the costals 

 there may be as few as 3 pits in the given line. On the distal ends of the costals there are 

 usually about 5 pits or grooves in the line mentioned. 



