PROTOSTEGID^:. 



195 



the fact that the costal plates were greatly reduced, so that the ribs were free from one another 

 to near their proximal ends. He supposed that he had found evidences, from the great length 

 of the costal plates from the origin of the rib-heads to the proximal border, that there were no 

 neural bones, and that the rib-heads were attacht to transverse processes. He was undoubtedly 

 wrong in both conclusions. Case had for examination only fragments of the ribs. He found 

 that the neural borders of the costal plates were digitated; and from the considerable extension 

 of these plates toward the midline from the origin of the rib-head, he concluded that there was 

 no room for neurals. He found that the costal plates extended along the ribs for about a third 

 of their length. Wieland had the opportunity to study a carapace, No. 1420 of the Carnegie 

 Museum, which showed a nearly complete series of ribs. Fig. 249, reproduced from Wieland, 

 shows this carapace as seen from below. He found the neurals to be present, but of papery 

 thinness and much crusht down on the neural arches. Contrary to what is found in most 

 turtles, the first rib is not turned backward against the second. It is relatively short. The 

 other ribs are about 35 mm. wide at the middle of their length. The costal plates, forming the 



FIG. 250. Protostega gigar. Plastron. X ,V After figure by Case. 



ent, entoplastron; hyo, hyoplastron ; hypo, hypoplastron ; xifh, xiphiplastron. 

 On the left is a number of peripherals. 



disk of the carapace, extend not more than one-third the length of the ribs. The total length 

 of the ten dorsal centra was 680 mm. The distance between the distal extremities of the 

 ribs of the fourth pair was 1060 mm. 



The nuchal bone of some members of the Protostegidae has already been discust. Cope 

 figured a bone which he regarded as the nuchal. It is certainly a median bone, but it appears 

 to be too small to be the nuchal, and may be the pygal. The nuchal may be supposed to 

 resemble that ofdrchelon, as figured by Wieland (Amer. Jour. Sci., v., 1898, p. 17). 



Of the peripherals Cope had 12, but the exact position of these could not be determined. 

 They were, moreover, greatly flattened by the pressure to which they had been subjected. 

 Some of these had only a single lamina, that rising to the ribs. Others possest, besides this, a 

 lamina that projected toward the bones of the plastron. The end of a rib was partly buried in 

 the lower surface of the upper lamina of each. The external border of each peripheral behind 

 the second was acute. The upper lamina was thin and its upper border terminated in digita- 



