258 FOSSIL TURTLES OF NORTH AMERICA. 
form nearly smooth lines. The hy poplastra meet along the midline a distance of 80 mm. The 
one of the right side has articulated by its anterior angle with the left hyoplastron for a short 
distance. ihe free border of the left hypoplastron is present from the inguinal notch to 
xiphiplastral suture. Below, it presents an acute edge; but the bone thickens rapidly. Out- 
side of the notch a process is sent outward and upward to enter a pit between the seventh 
and eighth peripherals. “The abdomino-femoral sulcus crosses the hypoplastra a little nearer 
the anterior end of these bones. The region where we might expect to find the inframarginal 
scutes is broken away. The longitudinal median sulcus is not seen. 
The sculpture of the plastron is obscure, but there are evidences everywhere of shallow pits, 
and these were arranged, at least partly, in rows. 
There are preserved the distal ends of 2 costals, one of which is the sixth of the right side, 
while the other belongs to the left side. The sixth of the nght side (plate 38, fig. 2) is 38 mm. 
wide and 4mm. thick. It is attacht principally to the eighth peripheral, but also to the anterior 
angle of the ninth. On the outer surface are seen parts of the third and fourth costal scutes, 
and of the eighth and ninth marginal scutes. The other distal end of a costal, probably the 
fifth of the left side, is 43 mm. wide and 4 mm. thick. A fragment of a costal from about the 
middle of the length of the bone is 35 mm. wide and 6 mm. thick. This seems to indicate 
that the costals thickened considerably toward the neurals. 
Three successive peripherals of the right side are among the materials, the seventh, 
eighth, and ninth (plate 38, fig. 2). They are remarkable for their great height. The seventh 
measures 46 mm. along the free border; the eighth and ninth, each 49 mm. The height of 
the latter is 65 mm. aa he exterior surface of the seventh is convex, with the free edge flared 
slightly upward. The other two are not curved vertically, but are convex from front to back. 
The greatest thickness of the ninth is at the front border, 11 mm. The thickness is reduced 
upward to the sutural border, less rapidly downward to the sharp free edge. The eighth 
thickens rapidly toward the anterior end, where, at the middle of the height, the thickness is 
23 mm. The thickening is exprest on the inner surface as a ridge. The hinder and lower 
inner portion of the seventh is likewise thickened, but the front and upper part is excavated 
by the visceral chamber. Above the thickened ridge, on the inside of each of these peripherals, 
is seen the mouth of a deep pit which received the free end of a rib. Two of these rib-ends 
are yet in place, one belonging to the sixth costal. 
Between the seventh and eighth peripherals, on their inner side (plate 38, fig. 3), there is 
a deep pit, excavated principally in the seventh, which has received ie extremity of the 
etinal process of the hypoplastron. The latter did not rise to the height of the costal plates. 
From the front of the pit a sharp ridge runs downward and forward. This has its inwardly 
projecting edge broken away, but it quite certainly furnisht an articulation with the outer 
border of the ‘hy poplastron. The seventh peripheral of the left side is present, but furnishes 
no additional information. A more posterior peripheral, possibly the eleventh, has the exterior 
surface considerably more concave vertically than that of the more anterior ones, showing 
that the hinder border of the carapace was flared upward. Plate 
37, fig. 10, shows a fragment of a bridge peripheral with a portion 
of the end of a rib inserted in a furrow. Text-fg. 321 represents 
a section of a bridge peripheral showing that there was a sharp 
keel along the side. 
As will be observed from the plate, the hinder marginal scutes 
extended up on the costal plates. The ninth marginal has a width 
fore and aft of 52 mm., and a height of 85 mm. _ Its upper border 
displays some of the irregularities so common among the Der- 
matemydida. On the costal supposed to be the Gah of the left side the sulcus bdteeen the 
marginal and the costal scutes runs lower down. 
Fic. 321.—Zygoramma stria- 
tula. Section across a 
bridge peripheral. 3. 
The outer surface of the costals and peripherals is sculptured into low ridges and shallow 
grooves. These are mostly directed from the front of the bone backward, but often also 
slightly downward. The downward inclination is seen especially on the more posterior bones. 
The length of the carapace of this specimen is estimated at 380 mm. 
