32 THE OSTEOLOGY OF THE REPTU^ES 



teeth and is more or less inconstant, absent in Sphenodon and most 

 Chelonia. It is a thin bone and forms the cover to the Meckelian 

 groove, whence the name opercular often given to it. Primitively 

 (Figs. 15-18) it formed a large part of the inferior border of the man- 

 dible anteriorly, appearing on its outer face, but in all late reptiles it 

 is restricted to the inner side. It enters into the mandibular sym- 

 physis in most long-jawed reptiles, probably an acquired character. 



The prearticular (goniale) {pa), recognized only within recent 

 years, is a thin bone, articulating with the articular behind, the angu- 

 lar below, the coronoid and splenial in front, forming the hind border 

 of the posterior inferior foramen and the lower margin of the Meckel- 

 ian orifice. It was present in all early reptiles (Figs. 16-18), and re- 

 mains a separate bone in the modern turtles (Fig. 31 e) and young 

 Sphenodon (Fig. 60). It was present in the Dinosaurs, Plesiosauria 

 (Fig. 25 a), where it was first named, Ichthyosauria, and doubtless 

 many other extinct reptiles. It is fused with the articular in the 

 Squamata (Fig. 55 b), extending far forward in the mosasaurs (Fig. 

 58), ensheathed by the united angular and coronoid, splenial and 

 dentary. It is apparently wholly absent in the Crocodiha (Fig. 69). 



The articular, the only cartilage bone of the mandible, forms the 

 cotylus, in whole or part. Distinct in all early (Figs. 16-18) and 

 many later reptiles, it may be indistinguishably fused with the pre- 

 articular or surangular. Believed to be the malleus of the mam- 

 malian ear. 



Openings in the mandible 



Aside from the large opening for the entrance of nerves and blood- 

 vessels [and jaw muscles] at the posterior upper part of the man- 

 dible (Figs. 16-18), there are in the early reptiles one or two smaller 

 openings through the inner wall: the posterior one just in front of 

 and below the anterior end of the large orifice, between the coronoid, 

 angular, prearticular, and splenial, is still present in the crocodilians 

 (Fig. 69 d). a large perforation of the outer wall of the mandible, 

 between the angular, surangular, and dentary, is very characteristic 

 of most Crocodiha (Fig. 69 c), Theropoda (Fig. 70 a), Phytosauria, 

 and Pseudosuchia (Fig. 65 a, e). 



A foramen posteriorly, between the prearticular and angular, is 

 for the passage of a track of the chorda tympani nerve. 



