THALASSEMYDID&. 139 
type specimen comprises all the peripherals of the left side, except the third, tourth, fifth, and 
eleventh; the second, fourth, eleventh and half of the ninth of the right side; the first left 
costal, lacking the distal end; the distal end of the first right costal, and many fragments of 
other costals; two neurals and a portion of a suprapygal; some fragments of the plastron; 
and two vertebra. Cope mentions five vertebrz. 
An estimate, based on the dimensions of the peripherals, shows that the carapace was 
probably slightly shorter than that of the type of 
O. gtbbi; that is, it was about 740 mm. long in a 
straight line. 
Professor Cope stated that this species is 
abundant in the upper bed of Cretaceous green- 
sand at several localities in New Jersey, but the 
present writer knows of but one other specimen. 
The nuchal bone is missing, but its form and 
proportions were probably similar to those of the 
nuchal of O. emarginatus. The following table 
presents the dimensions of the peripherals (plate 
26, fig. 2; text fig. 155). The length is taken 
along the free border. The height is the distance 
at the anterior end of the bone, from the free 
border to the costal border. The thickness is the 
greatest taken at the anterior end of the periph- 
eral. Where a definite inner face presents itself, 
First and second peripherals and first costal of type. as from peripherals 4 to 8, the thickness is 
X}. First peripheral at the right. replaced by the dimensions of these faces. The 
dimensions of the hinder end of any bone are the same as the dimensions of the front end of 
the succeeding bone. 
The free borders of the first (fig. 156) and second (fg. 157) peripherals are thick and 
obtuse. On the fourth there is a subacute, slightly upturned free edge, which may be traced 
forward as an obtuse keel, even to the first peripheral. On the fourth peripheral (fig. 158) 
the upper and the lower faces form an angle with each other greater than a right angle; but on 
the sixth (fig. 159) this is already less than a right angle; and more posteriorly the two faces 
E become more nearly parallel (figs. 160, 
Width. 161). The upper faces of the four ante- 
rior peripherals are irregularly convex; 
those of the sixth and seventh are some- 
what concave; those of the others are 
Fic. 155.—Osteopygts erosus. 
Peripheral. Length. Height. | Thickness. 
Lower face. Inner face. 
I 83 61 21 
5 81 so 5A nearly plane. 
3 : st 31 - In the hinder end of the second 
is a - a e peripheral (fig. 157) is a deep pit for the 
6 83 654 i 57 47+ anterior prolongation of the hyoplas- 
i 85 ze ‘Y = tron. Cope thought that this pit was 
5 ak eae 8 it * for the rib-end of the first costal. The 
10 95 97 17 c - pits in the greater number of the periph- 
8 got 105+ 7 erals are circular in section. Those of 
= 7 the ninth, tenth, and eleventh are fat- 
tened. In the last-mentioned cases the pits produce notches in the upper faces of the periph- 
erals. The anterior end of the inner face of the eighth peripheral is deeply excavated for the 
posterior outer angle of the hypoplastron. The inner face of the seventh is considerably 
excavated for the same purpose. Several shallow pits appear along the lower borders of the 
inner face of the sixth and seventh peripherals for digitations of the plastron. There was no pit 
in the eleventh peripheral. This bone had a thickness of 18 mm. where it joined the pygal. 
As regards the connection of the peripherals with the costals, we appear to have the fol- 
lowing: The first and second peripherals were closely sutured with the first costal. ‘The distal 
end of this costal shows that at least the anterior half of the third peripheral was sutured with 
it. The anterior end of the upper border of the fourth costal is smooth; the posterior two- 
