THE SKULL OF REPTILES 45 



vomer and the pterygoids. The pterygoids also usually meet in the 

 middle, though separated in the Trionychoidea by the basisphenoid. 

 The palatines also often meet for a short distance below and in front 

 of the internal nares, forming a rudimentary secondary palate. 



The temporal region primitively was wholly roofed over, and yet is, 

 in some marine turtles, by the large postorbital, quadratojugal, and 

 squamosal. Usually it is more or less exposed by the emargination 

 of the roof from behind or below, or from both sides; and the squamo- 

 sals and quadratojugals may even become vestigial in the process as 

 in the terapenes. The quadrate is always large, its ear-cavity some- 

 times wholly surrounded by bone. The stapes is slender. The con- 

 dyle is largely formed by the exoccipitals, in some wholly so. It re- 

 mains cartilaginous in the Dermochelyidae, as in some cotylosaurs. 

 The paroccipital remains free throughout life. 



The mandibles have a large, free prearticular, usually but incor- 

 rectly called the splenial; the splenial is rarely present (Emydura, 

 Toxochelys, etc.) . Both upper and lower jaws are encased in a cutting 

 horny sheath, and are without teeth. Small teeth on the pulatal 

 bones are known to occur only in Stegochelys, a Triassic genus. 



