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 fora rib. Itis 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 lamine, the uppermost of which is the broader. All these peripherals 
are closely joined together by means of coarse sutures. 
Wieland figures ( (Mem. Carnegie Mus., 11, p. 282) a portion of a nuchal bone and the first 
and second peripherals. The figure i 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 
Fic. 251.—Protostega gigas. Right shoulder-girdle and limb; dorsal view. Xs. 
c, centrale; cor, coracoid; hum, humerus; int, intermedium; I-V, the digits; met, metacarpals; phal, first row of 
phalanges; p/, pisiforme; pcor, procoracoid process; rad, radius; u/, ulna; uln, 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 1s 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 of the hyoplastra (fg. 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 a 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. 
The xiphiplastra are remarkable bones. Instead of extending backward and gradually 
inward as they do in the Cheloniidz, just behind their Aciculaven 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 ail fhe. Protostegidz, are modified for permanent residence 
on the seas, 
