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THALASSEMYDIDA. 145 
All the bones of the carapace here described are plentifully furnisht with pits. These vary 
in size from I to 10 mm. in diameter. That some of them are not the result of disease or 
parasites can hardly be afhrmed, but they must have been produced during the life of the 
individual. i 
The plastron may not have belonged to the individual that furnisht the peripherals; but 
the size befits it. In form it agrees with that of O. gibbi. At the narrowest portion of the 
bridge the hyoplastron and hypoplastron are each 67 mm. wide. As nearly as can be deter- 
mined, the width of the plastron from side to side was about 480 mm. There was evidently 
a fontanel inclosed by the two plastral bones mentioned and the peripherals. There was 
another at the crossing of the median and the hyohypoplastral sutures. There was probably 
still another at the midline just in front of the xiphiplastrals. 
The thickness of the bones at the bridge is 20 mm. 
The femur lacks the distal end. In form it resembles that of supposed O. sopitus, as 
figured by Cope; but the size is greater. The diameter of the shaft is 18 mm. 
No. 2216 of the American Museum of Natural History comes from some unknown locality 
of the Cretaceous greensand of New Jersey, and forms a part of the Cope collection. It 
Fics. 169-171.—Osteopygis borealis. Portion of carapace and lower jaw. 
169. Front of carapace of specimen regarded by Cope as 0. sopitus. No.2351 A.M.N.H. c. p. 1, first 
costal plate; nu. p, nuchal plate; nu.s, nuchal scute; per. 1, first left peripheral. XX #2 
170. Lower jaw, seen from above. No. 2216 A.M.N.H. X43 
171. Section along symphysis of same jaw. <4 
is referred confidently to the present species, altho some differences may be observed. 
It furnishes peripherals three, four, six to ten of the left side, five and eight of the right side; 
the eleventh of one side or the other; the outer end of the left hypoplastron; and a large part 
of the lower jaw. The latter is here described and figured. 
Fig. 170 represents the jaw as seen from above. Fig. 171 is a section along the symphysis. 
The length of the symphysis is 62 mm. The thickness at the hinder end of the symphysis 
is18mm. This is gradually reduced forward. The symphysis extends backward about 9 mm. 
behind the line joining the mental foramina. The width at the mental foramina is 92 mm. The 
crushing surface is broad and flat, rising at the sides to the cutting-edges. For a distance of 
about 32 mm. from the tip the cutting-edges are acute and are directed outward; they then 
become obtuse, as is shown by section. There certainly was no upturned beak at the tip of the 
jaw. Along the inside of the rami and behind the symphysis is a deep groove. The lateral 
halves of the jaw are solidly co-ossified, but there remain distinct traces of the suture. 
Osteopygis borealis differs from O. emarginatus, O. gibbt, and O. robustus in not having 
the peripherals between the second and the eleventh joined by suture with the distal ends of 
the costals. From O. erosus it differs in having bones of much thinner and lighter construction. 
The anterior peripherals are, relatively to their length, of less height. In O. borealis the pits for 
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