TRIONYCHID^E. 



55 



like No. 5917, lacks the median portion of the nuchal and the anterior neurals. Furthermore, 

 the sculptured layer has been split off the nuchal. The length of the carapace was close to 

 420 mm.; the width, 360 mm. The sixth neural was not completely developt and it permits 

 the costals of the sixth pair to join behind it. The seventh neural is only 15 mm. long and 10 

 mm. wide, and it is wedged in between the sixth and seventh pairs of costals. 



The nuchal had a width of 60 mm. from front to back, at a short distance from the midline. 

 It is evident from the right fragment of the nuchal and the right first costal that there was 

 between the nuchal and the first costals, one on each side of the first neural, a pair of fontanels. 

 Similarly placed fontanels are found in Platypeltis leucopotamica . In A. guttatus these fontanels 

 were about 25 mm. fore and aft and at least 20 mm. from side to side. Between the fontanels 

 of No. 5993 is a small triangular plate of bone which may represent the preneural. On account 

 of this bone, this species is referred to the genus Aspideretes. Without this it would fall into 

 the genus Amyda. 



This specimen is nearly devoid of the smooth marginal band of interlaced bony fibers so 

 conspicuous in No. 5917. 



As stated above, Professor Cope furnisht a brief description of a nearly complete carapace 

 obtained by him in the Wasatch beds of New Mexico and referred by him to this species 

 (Wheeler Survey, iv, p. 46). According to this description the nuchal underran the first 

 costal. This is not the case with the specimen described above and it is concluded therefore 

 that the Wasatch specimen does not belong to the same species as Leidy's type. 



Aspideretes ellipticus sp. nov. 

 Plate 95, figs. 1-3; teit-figs. 665, 666. 



The description of this species is based on No. 1117 of the American Museum of Natural 

 History, a specimen which was obtained in the Bridger beds of northeastern Utah by the 

 American Museum Expedition of 1893. 



The carapace (plate 95, fig. I; text-fig. 665) is complete; the plastron is wholly missing. 

 In form the carapace is more elongated than usual, rounded in front, only slightly truncated 

 behind. The borders, especially the anterior and lateral, are sinuous. From side to side the 

 carapace is gently archt. The total length is 400 mm.; the width, not regarding the convexity, 

 is 305 mm. The greatest width is at the middle of the length, and from this the margin pro- 

 ceeds with similar curves forward and backward. The thickness of the costals is about 9 mm. 

 at the sutures, but somewhat greater thru the ribs. Along the sides of the carapace the upper 

 and lower layers of the bones project beyond the median layer, thus producing a groove. The 

 lower layer extends far out on the distal ends of the ribs. 



Behind the nuchal there is a preneural, followed by 6 neurals. The table herewith presents 

 the measurements of these bones. 



The forms of these bones are shown in the figures. 



The nuchal has an antero-posterior extent of 50 mm. and a lateral extent of 162 mm. Its 

 outer extremities overlap the projecting ends of the ribs of the first costals. A band in front 



and at the extremities is not occupied by the sculptured 

 layer of bone. The anterior costal bone is large, having a 

 width at the proximal end of 67 mm. The free portions of 

 most of the ribs project beyond the margin of the carapace at 

 least 40 mm. The costal plates of the sixth, seventh, and 

 eighth pairs meet along the midline. Those of the eighth pair 

 are well developt. 



The sculpture of the carapace (plate 95, figs. 2, 3) con- 

 sists of large, rather shallow pits, which are separated by 

 ridges whose summits are rounded. Of these pits there are 

 4 or 5 in a line 20 mm. long. They are largest on the hinder 

 half of the shell. There is only a slight tendency toward confluence into rows and furrows. 

 On the neural bones the pits are not so distinct as on the costals. 



The type specimen of A. ellipticus was buried in a matrix of rather coarse sandstone. 



A second and larger specimen of this species was collected during the summer of 1903 in 



the Bridger deposits of the western part of Grizzly Buttes of Wyoming. Its catalog number 



