THALASSEMYDIDjE. 



[ 45 



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 hardlv be affirmed, but they must have been produced during the life of the 

 individual. 



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 0. gibbi. At the narrowest portion of the 

 bridge the hyoplastron and hvpoplastron 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 hyohvpoplastral 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 0. sopitus, as 

 figured bv 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 



Figs. 169-171. Osteopygis boreahs. Portion of carapace and lower jaw. 



169. Front of carapace of specimen regarded by Cope as . 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. X 5 



170. Lower jaw, seen from above. No. 2216 A. M. X. H. Xi 



171. Section along symphysis of same jaw. X 



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 

 is 18 mm. This is graduallv 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 solidlv en-ossified, but there remain distinct traces of the suture. 



Osteopygis borealis differs from 0. emarginatus, 0. gibbi, and 0. robustus in not having 



the peripherals between the second and the eleventh joined by suture with the distal ends of 



the costals. From 0. erosus it differs in having bones of much thinner and lighter construction. 



The anterior peripherals are, relatively to their length, of less height. In 0. borealis the pits for 



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