522 FOSSIL TURTLES OF NORTH AMERICA. 
shallow sinus in the anterior border of the carapace. The rear is broadly rounded. The shell 
was considerably archt from the lateral borders to beyond the middle of the costal plates, the 
rise being about 82 mm. Along the middle of the back there is a rather deep depression. 
The nuchal extended on each side of the midline at least 152 mm., and was 37 mm. wide 
fore and aft, perhaps somewhat more at the midline. There was no fontanel between it and 
the costals of the first pair and the first neural. Also there was no preneural. The first neural 
is large, being 68 mm. long and 37 mm. wide posteriorly. The others diminish in size to the 
seventh and last. The second and third are cothn-shaped, the next three nearly quadrilateral, 
and the last heart-shaped. This last one is only 22 mm. long and wide, and it separates only 
partly the seventh costals. 
All the costals grow broader toward their outer ends, except the first and the last pairs. 
Those of the eighth pair are united in the midline by their entire width, and by their free 
borders they fern 125 mm. of the hinder margin of the shell. At their free ends the most of 
the costals are beveled off to a sharp edge, except where the rib projects beyond the costal 
plate. Posteriorly the edge of the carapace is more obtuse. Near their outer ends the costals 
are about g mm. thick at the sutural border, and 12 mm. thru the rib. How far the ribs 
extended beyond the borders of the costal plates can not now be determined. Near the outer 
ends of the costals the ribs occupy nearly half the width of the lower surface of the plates. 
The nuchal had a broad, smooth band along its front, and this band has continued around 
the greater part of the carapace, becoming very narrow behind. The sculpture (plate 100, 
hgs. 3, 4) consists of the usual pits and ridges, the pits being rather deeply imprest and the 
ridges abrupt. There are 2 or 3 pits in 10 mm. They are large and distinct in the median 
depression (fig. 3) of the shell and on the middle of the length ah the costals, smaller on the 
highest part of the carapace and on the outer ends (fg. 4) of ‘the costals. As the smooth band 
is approacht, there is a tendency in the ridges to break up into tubercles. In figs. 3 and 4 of 
plate 100 the upper edge of the figure is that directed toward the head of the animal. 
Besides the carapace, Cope figured the outer ends of the hyoplastral and hypoplastral 
Ais of the right side; but the same bones of the left side, nearly complete, are present (fg. 
677) Lhe length of the suture joining them is 185 mm. The antero- posterior width of fhe 
ears at the inner end was about go mm.; that of the narrowest part, across the 
bridge, 35 mm. The antero-posterior extent of the inner border of the hypoplastron, exclusive 
of the processes, was 95 mm., that of the narrowest part, 28 mm. 
The thickness of the plastron at the inner ends, between the ridges of bone which ter- 
minate in the digitations, is g mm. Thru the thickest part of the bridge the bone measures 
15 mm. 
The lower surface of the bone is ornamented, next to the outer end of the bridge, with 
sharp ridges running antero-posteriorly. At the inner ends of the bones the surface is covered 
with sharp tubercles. The remainder of this surface is merely roughened. 
Amyda concentrica (Cope). 
Plate 98, figs. 7-10. 
Trionyx concentricus, Cope, Palwont. Bull. No. 1, 1872, p. 461; Proc. Amer. Philos. Soc., x11, 1873, 
p. 461; U.S. Geol. Surv. Terrs., 1872 (1873), p. 617; Vert. Tert. Form. West, 1884, pp. 118, 120, 
plate xvi, figs. 3-6.—Hay, Bibliog. and Cat. Foss. Vert. N. A., 1902, p. 454. 
Amyda concentricus, Hay, Amer. Geologist, XXXv, 1905, p. 336. 
The type of this species consists of 4 fragments of costal plates, 3 of which belong to the 
proximal ends, and 1 to the distal end, of their respective plates. These were found in the 
Bridger deposits along Cottonwood Creek, Wyoming. In his earliest description Professor 
Cope states that the remains of this species are common in the Bridger beds; but the present 
writer knows of no specimens except the types. These are now in the American Museum 
of Natural History and have the catalog number 1049. 
The animal to which the type costals belonged was a small one, probably about 160 mm. 
in length of carapace. The average width of the four costals is 19 mm. The thickness thru 
the sutural borders is 3 mm.; thru the ribs it is 4 mm. The sculpture appears to furnish 
characters for the identification of the species. On the proximal ends of the costals the pits 
