TRIONYCHID. 52.7 
The pits of the plastral bones 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 4 xestus byssinus (Axestemys 
byssina). here is, however, no reason for thinking that the present species is identical with 
Cope’ Ss 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 Platypeltis 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 4 xeste mys byssina; the loses diameter of the former being 8 mm., that 
of the latter 17 mm., while the length of the humerus of 4xestemys appears not to have been 
much greater than shat of the present species. 
This species is at once distinguisht from 4. salebrosa by the greater development of the 
costals of the two hinder pairs ood 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 4. egregra. Comparison with 4. concentrica is required, 
5 
Amyda mira sp. nov. 
Plate 105, fig. 1; plate 106; text-figs. 684-686. 
Under the above name 1s described a trionychid which was discovered by the American 
Museum party of 1905, at Grizzly Buttes, Wyoming g, 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. 1) is about as wide as long, broadly 
rounded behind, more sharply rounded in front, a of considerable convexity. The length 
ae Sees in a straight line is 263 mm.; the extreme width is 252 mm.; 
Neural. | Length. | Width. 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 
I 44 24 
2 31 23 with the upper surfaces. 
: os 2 The nuchal is 27 mm. wide, 134 mm. from extremity to 
5 24 18 extremity, and 97 mm. from its union on the free border with 
= hs 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 § pits or grooves in the line mentioned. 
