THAKASSF.MYDID^:. 



13.3 



and the eleventh, have each a pit in the inner face to receive the end of" a rib extending beyond 

 the costal plate. 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 

 marks one difference between this species and 

 0. emarginatus, the type of the genus. The 

 inner face of the second peripheral has a large 

 pit for the reception of the outer anterior exten- 

 sion of the hyoplastron. The inner face of the 

 third, fourth, fifth, sixth, and seventh periph- 

 erals is broad and longitudinally excavated. 

 These bones have thus three borders, the outer, 

 or free border; the upper border, which articu- 

 lates with the costals; and the inferior border, 

 which comes into contact with the plastron. In 

 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 

 upper surface. 



The pygal bone measures about 85 mm. along the free border 

 and has a height of about 75 mm. 



The sulci between the various scutes of the carapace are 

 rather deeply sunken. The dimensions of the vertebral scutes 

 are given in the table. 



The hinder border of the fifth lies on the posterior supra- 

 pygal. The supracaudals join along the midline a distance of 

 70 mm. and each is 40 mm. wide. The costo-marginal sulci lie wholly 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. 



Vertebral 



Width. 



145. 146. 



FIGS. 144-146. Osteopygis gibbi. Plastron and limb bones of type. 



144. Plastron. Xj. ab, abdominal scute ; an, anal scute; ent, entoplastron ; efi, epiplastron; /,/, median and 



posterior fontanels ; fern, femoral scute ; hy/>, hyoplastron ; itifmf, supposed inframarginal scute ; pec, pec- 

 toral scute; xiph, xiphiplastron. 



145. Humerus, dorsal surface. Xj. 146. Femur, dorsal surface. Xj. 



