THE SKULL OF REPTILES 1 5 



or fused with the premaxillae in the Mosasauria (Fig. 54 c) ; sepa- 

 rated by the premaxillae in the plesiosaurs, pterosaurs (Fig. 72), and 

 some lizards (Fig. 56), and probably absent in some of the former 

 (Fig. 46) . They do not enter into the formation of the nares in the 

 Rhiptoglossa (Fig. 55 d), but surround them in the Phytosauria 

 (Figs. 66, 67). Very large in the Ichthyosauria (Fig. 50), they also 

 articulate with the large postfrontals. They are often fused in the 

 midline. 



Frontals (fr). Primitively (Figs. 2, 4, 22, 23, etc.) articulating with 

 nasals, prefrontals, postfrontals, parietals, and ethmoids, often form- 

 ing the middle of the upper margin of the orbits. 



Always present and not varying much in their relations. In the 

 plesiosaurs (Fig. 46), pterodactyls (Fig. 72), and some lizards, they 

 articulate directly with the premaxillae (Fig. 56), and in some of the 

 former are separated externally in the middle. Often fused in mid- 

 line (Figs. 68, 69). 



Parietals (pa). Primitively (Figs. 2, 4) articulating with frontals, 

 postfrontals, intertemporal, supratemporals, tabulars, and inter- 

 parietals; below with the supraoccipital, epipterygoids, postoptics, 

 and prootics. 



In the absence {e. g., Figs. 31 b, 33, 44, 45, etc.) of the intertem- 

 poral and supratemporals, the parietals articulate directly with the 

 squamosals and postorbitals; in the Chelonia (Fig. 30) and Croco- 

 dilia, also directly with the pterygoids. Fused in most late reptiles 

 (e.g., Figs. 69, 72). 



Interparietals (ip). Primitively (Figs. 2 a, 4, 22) back of the 

 parietals on the superior surface of the skull, articulating with pari- 

 etals, tabulars, and supraoccipital. 



In the later Cotylosauria (Fig. 9), most if not all Theromorpha 

 (Figs. ^^, 42 d), some Therapsida (Figs. 44 a, d, g), they are situated 

 on the occipital surface and are usually unpaired. A vestige, sup- 

 posed to be these bones, occurs in some Crocodilia, originally named 

 dermosupraoccipitals. They do not help form any part of the cere- 

 bral wall. Unknown or doubtful in other reptiles. 



Lacrimals (la). Primitively (Figs. 2 b, 3, 22, 23, etc.) large, ex- 

 tending from orbit to nares, articulating with prefrontals, nasals, 

 septomaxillae, maxillae, and jugals. 



In the latest Cotylosauria {Procolophon) , most Theromorpha 



