Page:The Osteology of the Reptiles.pdf/63

Rh 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 squamosals and quadratojugals may even become vestigial in the process as in the terapenes. The quadrate is always large, its ear-cavity sometimes wholly surrounded by bone. The stapes is slender. The condyle is largely formed by the exoccipitals, in some wholly so. It remains cartilaginous in the Dermochelyidae, as in some cotylosaurs. The paroccipital remains free throughout life.

The mandibles have a large, free prearticular, usually but incorrectly 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 palatal bones are known to occur only in Stegochelys, a Triassic genus.