206 



FOSSIL TURTLES OF NORTH AMERICA. 



From the front of" the orbit to the quadrate the free borders of the maxilla, the jugal, and 

 the quadratojugal form a nearly straight line. Above the orbit the prefrontal meets the 

 postfrontal, thus excluding the frontal from the orbital rim. The relations of the bones behind 

 the orbit are in general like those of Protostega. In describing the quadratojugal of Protostega 

 Case appears to have inverted the bone and this has led Wieland to suppose that the bone 

 mentioned and others of the region are different in Archelon. Wieland's figure of the skull 

 of Protostega shows that the region in question is the same in both genera. The squamosal is 

 figured by Wieland as sending a process upward to meet the parietal. Probably the same 

 occurs in Protostega. The supraoccipital is represented as very short, but there is probability 

 that it was longer. The width across the quadrate region is about 350 mm. 



Various interesting structures appear on the palatal surface of the skull (figs. 264, 265). 

 As in Protostega, the basioccipital was small. The basisphenoid too appears to have been 

 small, but it is not well known. The pterygoids are extremely narrow. The exact relationships 



I IG. 166. Irchelon ischyros. Shoulder-girdle, humerus, and a cervical vertebra. Xi'n. 



cer.v, third ur fourth cervical vertebra; the figure on the right giving front view, that on the left a side view. 

 cor, coraccid; hum, humerus; sea/), scapula. 



of the palatines are not known. The hinder end of the vomer is missing, but it is probable that 

 it interposed between the palatines. The choanas were placed far forward, immediately below 

 the external hares, and were not underfloored by the surrounding bones. The great crushing- 

 surface of the upper jaw was developt mostly on the premaxillaries, an unusual thing among 

 the turtles. On the maxilla this surface extends backward only to opposite the choanae. 

 Behind these, the free border of the maxilla is narrow and weak. The crushing-surface was 

 therefore about 240 mm. long and about 140 mm. wide across the vomer. The anterior portion 

 of the vomer formed a boss in the center of the crushing-surface. 



The lower jaw (fig. 263 | was massive. Its lateral halves were not co-ossified, thus differing 

 from those of Protostega. The symphysis has a length of 120 mm., the total length of the jaw 



