THALASSEMYDID&. 147 
this as a mistake. After passing about 6 mm. the sulcus between the nuchal scute and the 
first marginal, the sulcus between the nuchal and the vertebral scutes suddenly becomes very 
shallow, but the writer believes that it continues on in its usual position. 
A considerable part of the suprapygal (fig. 173) is missing. Cope’s statement that it is 
only 2 inches and 4 lines wide is an error or meant to apply to some other bone. The bone in 
question is at least 110 mm. wide. It occupies an area that in O. gzbbi is occupied by two bones, 
——— = but a close examination shows a line running 
Gength |! Height |e Width. from the upper angle of one eleventh peripheral 
Peripheral. | 2 border. at sulcus. y | face. Innerface, t0 that of the opposite side, along which 2 bones 
a= | is i — have co-ossified. The angulation of the upper 
; p : 7 border of the eleventh peripheral shows that 
5 75 49 15 y there were two suprapygals. Itis probable that 
3 70 . 2 7 the sutural edge of the eleventh peripheral joined 
es s as a the anterior suprapygal instead of the eighth 
6 86 51 41 37 costal plate. The dimensions of the peripherals 
g x Pe a 2 are shown in the table. 
A 3 52 és 7 The sixth peripheral (figs. 174,175) presents 
10 - Be Ee three faces, an upper concave, a lower convex, 
a ae | 95 - and an inner irregular. The upper and lower 
faces meet at the acute free border. In the 
inner face is a large, somewhat flattened pit for the end of a rib. Its mouth occupies one- 
half the length of the face. Along the lower border of the face are two or three shallow pits 
for digitations of the hypoplastron. 
As we proceed forward from the sixth peripheral the free border becomes less acute, 
until, on the third, it is obtuse and the upper, now convex, face rounds into the lower. On the 
second and first peripherals (figs. 176, 177) the lower face becomes the obtuse free border 
of the bones. The inner faces of 
‘ the fourth and the third contain pits 
‘ for the corresponding rib-ends. The 
= inner face of the second has a large 
excavation which received the anterior 
outer process of thehyoplastron. Fig. 
176 represents the first, second, and 
third peripherals of the right side. 
The upper border of the first and 
b the anterior half of that of the second 
peripheral had a sutural articulation 
with the first costal (fig. 176). The 
thickness of these borders is 6 mm. 
The hinder half of the upper border 
of the second and the upper borders of all the other peripherals to the eleventh are smooth. 
The upper border of the eleventh appears to have articulated with the suprapygals, as 
already stated. 
Passing backward from the sixth peripheral, the upper and lower borders of all the 
peripherals broaden and become flatter. The inner face narrows, and finally on the eighth 
(figs. 178, 179) and succeeding peripherals, curves into the lower face. Each of these periph- 
erals, except the eleventh, has a pit for a rib-end. The hindermost peripherals are thin, the 
ninth being 14 mm. thick, the eleventh about 10mm. The flattening of the rib-pits begins with 
the sixth peripheral. The pygal is represented by fig. 173. Fig. 180 shows one lateral border. 
There are present many fragments of costal bones, but no complete costal. The remains 
seem to show that the carapace was rather flat. The thickness of the costals near the neurals 
was about 6 mm.; near the distal ends, about 5 mm. The rib-heads were strongly developt. 
Distally, each rib projected beyond the costal and entered the pit in a corresponding periph- 
eral. The edges of the costal plates approacht closely the upper borders of the peripherals. 
On the proximal ends of two costals the costo-vertebral sulci run along about 35 mm. from 
the neural border. Another fragment shows that the vertebral scute was strongly angulated 
d 
Fic. 174.—Osteopygts platylomus. Sixth left peripheral 
of type. X4. 
a, from above; b, hinder end; c, anterior end. 
