THALASSEMYDID. 133 
and the eleventh, have each a pit in the inner face to receive the end of a rib extending beyond 
the costal ea In the hinder peripherals these pits are flattened; also they produce notches 
in the upper face of a number of the hinder peripherals, as they do in O. borealis. This 
deficiency in the upper wall of some of the pits 
| it eee Width. ae marks one difference between this species and 
Perip eral. vength. Thickness. (Olte 0 : : = pay 
em cretenice O. emarginatus, the type of the genus. The 
inner face of the second peripheral has a large 
; ae be 7 pit for the reception of the outer anterior exten- 
iy 32 Ga a of sion of the hyoplastron. The inner face of the 
3 75 os . - third, fourth, fifth, sixth, and seventh periph- 
76 Te} wa 
2 A av a erals is broad and longitudinally excavated. 
6 85 62 4 These bones have thus three borders, the outer, 
| z 92 - - ; or free border; the upper border, which articu- 
s 95 e 26 e . re i 
i as ee Rae i. lates with the costals; and the inferior border, 
| 10 90 11 14 which comes into contact with the plastron. In 
it 83 110 
the anterior end of the eighth there is a cavity 
into which the outer posterior process of the 
hypoplastron projects. The inner face of the peripherals behind the eighth becomes narrow 
and curves into the lower face. Thus, these hinder peripherals are thin and wedge-shaped. 
The dimensions of the peripherals are shown in the table herewith. 
The acute free border of the hinder peripherals may be traced forward on the more anterior 
ones, where it forms a keel separating the upper face from the lower. The peripherals, from 
the seventh to the eleventh inclusive, are slightly concave on the 
|Vertebral.| Length. | Width, | UPPer surface. ; 
Che pygal bone measures about 85 mm. along the free border 
| and has a height of about 75 mm. 
| ' be re The sulci between the various scutes of the carapace are 
3 | 133 115 rather deeply sunken. The dimensions of the vertebral scutes 
4 142 435 are given in the table. 
5 sid The hinder border of the fifth lies on the posterior supra- 
pygal. The supracaudals join along the midline a distance of 
7o mm. and each is 40 mm. wide. ‘The costo-marginal sulci lie w holly on the peripheral bones, 
close to their upper margins, except at the rear, as just noted. 
There are four pairs of costal scutes. 
The surface of the bones of the carapace is usually smooth; but in places there appear 
striations, which are directed outward and backward. ‘There are no pits. 
ME epi NN ar 
“yy 
i 
> 
| hypo 
146. 
Fics. 144-146.—Osteopygts gtbbi. Plastron and limb bones of type. 
144. Plastron. X$. ab, abdominal scute; am, anal scute; ent, entoplastron ; epi, epiplastron; f, f, median and 
posterior fontanels; fer, femoral scute; hyo, hyoplastron; infm?, supposed inframarginal scute; pec, pec- 
toral scute; x/ph, xiphiplastron. 
145. Humerus, dorsal surface. x}. 146. Femur, dorsal surface. +. 
