TRIONYCHID&. 525 
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 4. 
grangert all have much more elongated carapaces. 
Amyda egregia has a thicker carapace, a smoother sculp- 
ture, and a nuchal less extended laterally. In 4. scutum- 
antiquum the carapace is more elongated, the nuchal 1s 
furnisht with a smooth band, and the sculpture is every- 
where finer. In 4. radula the nuchal is nearly straight 
in front. 
It is 4. uintaénsis to which the present species 1s 
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 
having larger and 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. 
Fic. 680.—A myda salebrosa. Cara- 
pace of type.  $- 
Amyda? exquisita sp. nov. 
Text-figs. 681-683. 
This species 1s 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 1s 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. 681) 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, ts 
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 tog 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 
