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32 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 mandible anteriorly, appearing on its outer face, but in all late reptiles it is restricted to the inner side. It enters into the mandibular symphysis 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 angular below, the coronoid and splenial in front, forming the hind border of the posterior inferior foramen and the lower margin of the Meckelian orifice. It was present in all early reptiles (Figs. 16–18), and remains a separate bone in the modern turtles (Fig. 31 ) and young Sphenodon (Fig. 60). It was present in the Dinosaurs, Plesiosauria (Fig. 25 ), where it was first named, Ichthyosauria, and doubtless many other extinct reptiles. It is fused with the articular in the Squamata (Fig. 55 ), 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 Crocodilia (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 prearticular or surangular. Believed to be the malleus of the mammalian ear.

Aside from the large opening for the entrance of nerves and blood-vessels [and jaw muscles] at the posterior upper part of the mandible (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 ). A large perforation of the outer wall of the mandible, between the angular, surangular, and dentary, is very characteristic of most Crocodilia (Fig. 69 ), Theropoda (Fig. 70 ), Phytosauria, and Pseudosuchia (Fig. 65 ).

A foramen posteriorly, between the prearticular and angular, is for the passage of a track of the chorda tympani nerve.