Page:The Osteology of the Reptiles.pdf/37

Rh Squamata, and Ichthyosauria. Only in the Cotylosauria and some Theromorpha does it articulate with the pterygoids. In later forms it articulates with the paroccipital to a limited extent, supporting the head of the quadrate. In many Therapsida (Figs. 43, 44 ), but not in the more primitive Theromorpha (Fig. 33 ), it may articulate with the postorbital below as well as above the temporal opening. Its relations with the quadratojugal are also inconstant, lost in the Crocodilia (Fig. 69 ) and Predentata (Fig. 70 ). On the other hand, it may extend forward to unite with the maxillae in some plesiosaurs (Fig. 46 ). In the Squamata (Fig. 55 ), as most recent authors identify the squamosal, it articulates with the bones usually called the supratemporal and the postorbital (rarely excluded from it) and usually with the jugal.

Quadratojugals (qj). At the outer posterior side of the temporal region (Figs. 2, 3, 22, 33), overlapped by the squamosals, articulating in front narrowly with the jugals, behind with the quadrates.

The quadratojugals are relatively large in the primitive skull, sometimes forming a part of the articular surface for the mandible (Fig. 21 ). In the single-arched skull the quadratojugal tends to disappear. It is probably present in all Theromorpha, but is often confined chiefly to the posterior side of the quadrate (Fig. 42 ). It is absent in most Therapsida, the Sauropterygia and the Squamata. It enters into the boundary of the lower temporal opening only in the Crocodilia (Fig. 69), Phytosauria (Fig. 66 ), Pseudosuchia (Fig. 65), Pterosauria, Theropoda (Fig. 70 ), and some Predentata (Fig. 70 ), excluded in many Predentata (Fig. 70 ), as in all the other double-arched reptiles. It is very large in some Chelonia (Fig. 30 ), articulating with the postorbitals, as is also the case in the Crocodilia (Fig. 69 ).

Prevomers (pv). Paired bones on the palatal surface, articulating with the premaxillae in front, the pterygoids and palatines behind separating the internal nares; dentigerous (Figs. 6, 40 ).

Only in the Chelonia (Fig. 32 ) are the prevomers single, though sometimes fused in the Rhiptoglossa, Theropoda, and Theriodontia (Figs. 43, 44 ). They are edentulous in all known reptiles except the Cotylosauria (Fig. 6), some Theromorpha, perhaps, certain "Pseudosuchia," Diaptosauria (Fig. 63), and Squamata. Posteriorly