DERMATEMYDID&. 251 
of one side to those of the other amounts to 124°. Unfortunately the free border of the front 
lobe has not been preserved, so that it is impossible to determine the width of the lobe at the 
base; but it could not have been far from 100 mm. The notch for the entoplastron shows that 
this bone had a width of 42 mm. It appears to have been broadly rounded behind. While 
the thickness at the hinder border was about 17 mm., it was much thinner at the front. The 
thickness of the hyoplastron a short distance behind the entoplastron is 20 mm.; at the hinder 
border of the hyoplastron in the midline, 15 mm.; at the articulation with the ffth peripheral 
1omm. ‘The bone is therefore very thick and its tissue is dense and heavy. 
There is present a fragment of the free border of the hypoplastron, including a portion of 
the sutural border for the xiphiplastron. At a distance of 28 mm. from the free border the bone 
is 18 mm. thick. This continues to within about 20 mm. of the border, and then the bone is 
rapidly beveled off to a blunt edge. The lower surface of the bone curves upward to meet the 
bevel. 
The surface of the hyoplastron and of the fragment of the hypoplastron is very smooth, 
having apparently been polisht during the life of the animal. The proximal end of the first 
costal is preserved. Its neural border is concave for the first neural. Its hinder angle joined 
the second neural. Here the bone is 12 mm. thick. In front it is 8 mm. thick. On the under 
side are seen the base of the rib-head and articulation for the first rib. 
Two adjacent costals of the right side (plate 37, fig. 7), the fourth and the fifth, or the 
fifth and the sixth, are present. They are each 45 mm. in width at the proximal end. At the 
suture with the neurals they are 16 mm. thick; at a distance of 60 mm. from the neurals one 
has still a thickness of g mm. The capitula of the ribs are moderately developt. Little or no 
trace is left of the rb on the inner side of the costal plates. These costals are considerably 
archt, showing that, with the curvature of the plastron, the shell was high and vaulted. 
Fics. 311-313.—Agomphus petrosus. 
Peripherals of type. Xj. 
311. Section at anterior end of second peripheral. ; 
312. Section at anterior end of supposed eighth 
peripheral. 212 
313. Section across supposed first peripheral of oe 
left side. : 
Two anterior peripherals (plate 36, fig. 4), the second and the third of the right side, are 
connected with the outer anterior angle of the hyoplastron. They are thick and massive. 
The free edge of these peripherals is rounded like the edge of one’s hand, and at the anterior 
end of the second the border is slightly everted. Fig. 311 represents a section taken at this 
point. The thickness here is 15 mm. The second peripheral is 40 mm. in length along the 
free border and 40 mm. high. The third is 50 mm. long. The hinder border of the latter 
peripheral and the anterior lateral process of the hyoplastron furnish the articular border for 
the fourth peripheral; while behind this is a part of the articular border for the fifth periph- 
eral. Two of the hinder peripherals (plate 37, fig. 6), supposed by Cope to have been the 
eighth and the ninth, are at hand. The supposed eighth did not articulate with the hypoplas- 
tron as it appears to have done in Wieland’s 4. tardus and may therefore be the ninth. The 
fragment of costal attacht to it appears to be the fifth, since on it there is no trace of a descend- 
ing sulcus. These, with the distal end of the fifth costal, are figured on plate 37. Fig. 312 
presents a section across the anterior end of the supposed eighth. The free borders of these 
peripherals are thick and rounded like those of the anterior peripherals. The greatest thickness 
of the eighth is 20 mm.; but where the bone articulates with the costal the thickness is reduced 
to6mm. The ninth peripheral has the border considerably flared upward and more acute than 
further forward. Accompanying the type is another peripheral (fig. 313) which Cope has 
labeled as the first of the left side. On one side there is an excavation, as if for the nuchal. 
However, there are difficulties in regarding it as the first peripheral of this species One of 
these is the great thickness of the bone where it would come in contact with the second periph- 
eral, being 19 mm., whereas the corresponding border of the second peripheral of the right 
side is only 16 mm. thick. Again, as the upper surface approaches the free border, it curves 
