196 



FOSSIL TURTLES OF NORTH AMERICA. 



tions. We are indebted to Case for more exact knowledge of the peripherals. He had a series 

 of 8 of these in their natural order (fig. 250). The most anterior was supposed to be the second. 

 It is slender, concave along its free border, as if to make room for free movements of the limb, 

 and has no pit for a rib. It is 170 mm. long and only 30 mm. wide. The third is strong, broad, 

 and has a rib-pit. Its length is 170 mm. and its width 118 mm. The succeeding ones are longer 

 than wide and have 2 lamina?, the uppermost of which is the broader. All these peripherals 

 are closely joined together by means of coarse sutures. 



LWieland figures (Mem. Carnegie Mus., 11, p. 282) a portion of a nuchal bone and the first 

 and second peripherals. The figure is here reproduced (fig. 248). 



The hyoplastra and the hypoplastra were described by Cope as dermal bones that were 

 supposed to overlie the ribs. Baur's suggestion that these bones belonged to the carapace has 

 been mentioned. They had been comprest until the greatest thickness was only about 15 mm. 

 A plastron described by the present writer showed that these bones were really thick and heavy, 

 the thickness amounting to as much as 45 mm. in the case of the hyoplastron. The plastron 

 described by Case (fig. 250) was somewhat larger than the one last referred to and furnisht 

 representatives of all the elements except the epiplastra. The hyoplastra and hypoplastra are 

 bones of an irregularly triangular form, with all the borders digitated, except over the fore and 



Fig. 251. Protostega gigns. Right shoulder-girdle and limb; dorsal view. Xi. 



c, centrale; cor, coracoid; hum, humerus; int, intermedium; I-V, the digits; met, metacarpals; phnl, first row of 

 phalanges; pi, pisiforme; pcor, procoracoid process; rail, radius; ul, ulna; /n,ulnare; 1-5, carpals of second row. 



hinder limbs. From the anterior inner angle of the hyoplastron a thickening, appearing on the 

 lower surface as a low ridge, runs backward thru the articulation with the hypoplastron and 

 on backward to the xiphiplastra. From this ridge, in each direction, the thickness is reduced 

 until the borders are reacht. Evidently there were considerable spaces left between the plastral 

 bones and the peripherals. In the midline there was a great fontanel, which extended from 

 the entoplastron to the xiphiplastra. The union of the plastral bones with the peripherals was 

 a ligamentous one. 



As stated, the entoplastron was first described by Hay as a nuchal. Case figured a more 

 complete specimen as a nuchal. This was found lying with its wings overlapping the anterior 

 ends ol the hyoplastra 'fig. 250). It extended from side to side a distance of 599 mm. The 

 greatest breadth of the wings was 131 mm. From the midline of the bone there reacht back- 

 ward ;i spine which must have attained a length equal to that of the wings. The outline of this 

 bone was concave in front to middle of wings, then convex. The epiplastra are unknown. 



1 he xiphiplastra are remarkable bones. Instead of extending backward and gradually 

 inward as they do in the Cheloniidae, just behind their articulation with the hypoplastra they 

 turn directly inward at almost a right angle, to meet at the midline by an overlapping joint. 



The limbs of this species, as of all the Protostegidae, are modified for permanent residence 

 on the seas. 



