CIIELQNIA 



221 



prevomcrs. The vomer of the turtles under this theory is beheved 

 to be the real homologue of the mammaUan bone. The i")ahite is 

 always sHghtly, sometimes nearly wholly, underfloored, as in mam- 

 mals, carrying the internal nostrils far back in the mouth. In the 

 occipital region of the skull there is a separate bone on each side 

 called the paroccipital or opisthotic, which has been indistinguish- 

 ably fused with the exoccipital in all other reptiles except the ich- 

 thyosaurs since Triassic times. 



prnx 



ju. 



Fig. II 5; 



Fig. 116 



Figs. 115 and 116. — Trachemys. (From Hay) 



Fig. 115. — Skull from above: fr, frontal; jii, jugal; pa, parietal; paoc, paroc- 

 cipital; pfr, prefrontal; pof, postfrontal; pro, prootic; qu, quadrate; sq, squamosal; 

 soc, supraoccipital. 



Fig. 116. — Skull from below: alv, alveolar surface of maxilla; hoc, basioccipital; 

 bap, basisphenoid; cxoc, exoccipital; w.v, maxilla; pal, palatine; paoc, paroccipital; 

 pmx, premaxilla; pro, prootic; pt, pterygoid; qu, quadrate; qj, quadra tojugal; 

 sq, squamosal; vom, vomer. 



In the feet the numbers of phalanges — that is, the bones of the 

 free digits^ — are like those of mammals, that is, two in the first 

 and three in each of the other four digits. The land tortoises have 

 lost some of these, while the river turtles have either gained one 

 or two in the fourth finger and fourth toe. or else have enjoyed 

 an uninterrupted descent from the primitive reptiles which normally 



