TRIONYCHIDA;. 



525 



o.-^mj/ a salebrosa. 



Cara- 



The sculpture of this species presents a rather peculiar appearance, being coarse, irregular 

 and rough. The pits are irregular in size, form, in height of bounding walls, and in disposition 



with reference to each other. On the central portion of 

 the disk, including the greater part thereof, there are from 

 2 to 5 pits in a line 20 mm. long. Around the borders 

 of the carapace the pits are slightly smaller than else- 

 where, but often not well defined. Everywhere the walls 

 separating the pits are rather sharp and of varying height, 

 often being lifted up into sharp points, especially in the 

 angle formed by the junction of three pits. 



This species is sufficiently different from all others 

 yet found. Aspideretes guttatus, A. ellipticus, and A. 

 grangeri all have much more elongated carapaces. 

 Amyda egregia has a thicker carapace, a smoother sculp- 

 ture, and a nuchal less extended laterally. In A . scutum- 

 antiquum the carapace is more elongated, the nuchal is 

 furnisht with a smooth band, and the sculpture is every- 

 where finer. In A. radula the nuchal is nearly straight 

 in front. 



It is A. uintaensis to which the present species is 

 most closely related. It differs from the latter in having 

 the greatest width of the shell in front of the middle of 

 the length; in having a more convex front; and in 

 havmg , arger an( j more irregular pits . The types of 



the two species are of almost exactly the same length 

 and may therefore be more readily compared. A . scutumantiquum is longer than broad; 

 A. salebrosa is broader than long. 



Amyda? exquisita sp. nov. 

 Text-figs. 681-683. 



This species is represented in the American Museum of Natural History by portions of 2 

 individuals, which were collected in the Bridger beds of the Grizzly Buttes, Wyoming, by the 

 museum's expedition of 1903. Of these specimens, number 5923 is taken as the type. It con- 

 sists of the carapace (fragmentary but representing well the structure), the hyoplastra and 

 hypoplastra of both sides somewhat damaged, one cervical vertebra, portions of both humeri, 

 the left femur, portions of the pelvis, and various other leg and foot bones. Of the other 

 individual, No. 5943, there are present only the hyoplastron and the hypoplastron of the 

 right side. 



This species is markt among the species of the genus found in the Bridger beds by the 

 thinness of the shell and the coarseness of the sculpture. The total length of the carapace 

 (fig. 68 1 ) was very close to 375 mm.; the greatest width almost the same. Evidently the out- 

 line, both in front and behind, was slightly concave; laterally it is sinuous. The disk was 

 filled out nearly to the ends of the ribs. 



The thickness of the sutural border of the third costal plate, near the proximal end, is 

 6 mm.; but thinner toward the distal end of the plate. Thru the rib, in the middle of the 

 width of the costal, the thickness amounts to 9 mm. The sutural border of the eighth costal 

 is 7 mm. thick. 



The exact dimensions of the nuchal bone can not be determined, there remaining only 2 

 fragments; but evidently the lateral extent of the bone was close to 250 mm., and the fore-and- 

 aft extent, at the midline, very close to 50 mm. Fig. 682 represents a section of the bone along 

 the midline. The free border is reduced to a subacute edge. Seen from above, the nuchal 

 presents a smooth band along the anterior border. This is 13 mm. wide at the midline, 

 but increases to twice this width toward the distal ends. The maximum thickness of the bone 

 is about 13 mm. 



From the materials at hand it can not be certainly determined whether there was a pre- 

 neural bone; but probably this was absent. There are 7 neurals, the dimensions of which are 



