THALASSEMYDIDZ. 131 
width not far from the distal end. Its thickness is about 9 mm. It is interesting, because it 
shows plainly that it was articulated suturally with the contiguous peripheral. Fhe bone is 
traverst longitudinally by a deep sulcus. Its surface is pitted just like a costal of O. erosus. 
The neural figured by Cope (fig. 141) is present, as well as fragments of two others. The 
one figured is 55 mm. long and 36 mm. wide, deeply notcht in front, and crost behind by a 
transverse sulcus. It is probably. one of the more posterior neurals, the seventh or the eight. 
The surface of the anterior bones of the carapace is relativ ely smooth. The fifth peripheral 
is somewhat pitted, but the result probably of accident. 
The nuchal scute measures 60 mm. in front, 110 mm. behind, and 25 mm. fore and aft. 
The first marginal measures 52 mm. along the free border and 39 mm. across the end next the 
second marginal. 
The first vertebral had a width anteriorly of 185 mm. and a length of 82 mm. The second 
vertebral appears to have been about 75 mm. wide. No trace is seen of the costo-marginal 
sulcus on the upper border of the fifth peripheral; but doubtless on the sides of the shell the 
sulci followed closely the costo-peripheral sutures. On the eighth peripheral the costo-mar- 
ginal sulci lie from 12 mm. to 25 mm. below the upper border of the bone. 
The femur is represented by a portion of the proximal end; but the head and both trochan- 
ters are missing. The shaft has a diameter of 15 mm. 
The first specimen described by Cope bears the American Museum’s number 1344. It 
was an individual of almost exactly the same size as that regarded as the type. It furnishes 
the nuchal, the first right peripheral, the first, second, fourth, fifth left peripherals, the proximal 
half of the first left eect, some other unimportant costal fragments, a part of the right hyo- 
plastron, a part of the right hypoplastron, and the greater part of both xiphiplastra. Cope 
mentions additional peripherals, but they do not accompany the bones mentioned above, and 
have not been recognized in the Cope collection. 
That paragraph of Cope’s description which occupies the upper two-fifths of page 137 of 
the Synopsis may be regarded as a riddle; for in it Cope constantly compares the species he is 
describing with itself. The explanation appears to be the following: With the specimen, in 
Cope’s writing, is a label ‘ ‘Osteopygis platylomus, Cope, type: nr. Barnesboro, Gloucester 
Co; N. J.” At some subsequent time two pencil lines have been drawn through ‘ ‘platylomus.’ 
‘Where can be no doubt that the specimen is the one Cope describes as O. emarginatus; and 
there can be little doubt that the plastral bones furnisht the right hand portion of Cope’s 
fig. 39, said to be that of the plastron of O. platylomus. We may suppose therefore that at one 
time Cope regarded this specimen as: platylomus and intended to make it the ty pe of the 
species; but coming to see that it was O. emarginatus, a portion of the manuscript was 
transferred to the latter species without proper changes. 
The parts common to this specimen and the type of the species present no important 
differences. The second peripheral is 75 mm. long; 57 mm. at right angles with the free border 
and at the proximal end, and 21 mm. thick at the same end. The distal end has a large exca- 
vation for the process of the hyoplastron. The fourth peripheral is 67 mm. on the free border, 
38 mm. across the lower face. Most of the upper face is broken away. An obtuse keel 
separates the upper face from the lower. The inner face of the fifth was more excavated than 
in the same bone of the type; and, as Cope states, the pit for the end of the rib is much 
smaller. 
The bones belonging to this specimen which Cope appears to have employed in his fig. 39 are 
the hyoplastron, the hypoplastron, and xiphiplastron of the right side. The figure shows 
these bones and those of the type of O. platylomus as seen from above. A fontanel existed in 
the midline where it was crost by the hyohypoplastral suture. Where the plastral bones 
joined along the midline the sutures are coarse and jagged. ‘The xiphiplastron sent a strong 
process into the outer border of the hypoplastron and the latter sent one outside of this into the 
xiphiplastron. Just in front of this exchange of digitations the hypoplastron is 15 mm. thick. 
At this level the hinder lobe of the plastron was about 165 mm. wide. On the lower side of the 
hypoplastron the abdomino-femoral sulcus is seen in the position represented by Cope in his 
figure 39. Crossing the xiphiplastron the femoro-anal sulcus falls a little further forward 
than represented by Cope in the figure quoted. 
None of the bones of this specimen is pitted. 
