THE SKULL OF REPTILES 19 



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 b), but 

 not in the more primitive Theromorpha (Fig. 33 a), 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 c) and Predentata (Fig. 70 c). On the other 

 hand, it may extend forward to unite with the maxillae in some 

 plesiosaurs (Fig. 46 b). In the Squamata (Fig. 55 a), 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 (gj). At the outer posterior side of the temporal 

 region (Figs. 2 b, 3, 22, t^t^), overlapped by the squamosals, articu- 

 lating 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 b). 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 d). 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 b), Pseudosuchia 

 (Fig. 65), Pterosauria, Theropoda (Fig. 70 a), and some Predentata 

 (Fig. 70 Dj, excluded in many Predentata (Fig. 70 c), as in all the 

 other double-arched reptiles. It is ver>' large in some Chelonia (Fig. 

 30 a), articulating with the postorbitals, as is also the case in the 

 Crocodiha (Fig. 69 c). 



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 c). 



Only in the Chelonia (Fig. 32 b) are the prevomers single, though 

 sometimes fused in the Rhiptoglossa, Theropoda, and Theriodontia 

 (Figs. 43 c, 44 e). They are edentulous in all known reptiles except 

 the Cotylosauria (Fig. 6), some Theromorpha. perhaps, certain 

 "Pseudosuchia,'' Diaptosauria (Fig. 63), and Squamata. Poste- 



^ [But see footnote, p. 52. — Ed.] 



