284 THE VERTEBRATE SKELETON. 



are specially large in the Pterosauria and in some of the 

 Crocodilia and Dinosauria (fig. 35, 3). In some Pterosauria 

 they are confluent with the orbits. 



The premaxillae are usually separate, but sometimes, as in 

 some Ophidia (fig. 51, 1), Chelonia, Lacertilia (Agamidae), 

 and Dinosaurs (Ceratopsia) they are united. In the Dinosaur 

 Hadrosaurus they are exceedingly large and spatulate. In 

 the Rhynchocephalian Hyperodapedon they are drawn out into 

 a strongly curved beak. 



As regards the mandible, sometimes, as in most Rhyncho- 

 cephalia, Ophidia and Pythonomorpha, the rami have only a 

 ligamental union ; sometimes, as in Crocodilia, the Rhyncho- 

 sauridae and the majority of Lacertilia, they are suturally 

 united. In Chelonia (fig. 28, B, 12), and apparently in Ptero- 

 sauria, the two dentaries are completely fused together. The 

 sutures between the various bones of the lower jaw usually 

 persist, but in Ophidia those between the angular, supra - 

 angular, articular and coronoid are obliterated. There are 

 sometimes large vacuities in the mandible, as in Theromorpha, 

 Crocodilia, and some Dinosauria. In Iguanodon, Polyonax, 

 Hypsilophodon and Hadrosaurus among Dinosaurs the man- 

 dible has a predentary or mento-meckelian bone which, in 

 some cases at any rate, was probably sheathed in a horny 

 beak. 



The principal part of the auditory ossicular chain is formed 

 by a rod-like columella. The development of the hyoid appa- 

 ratus varies, and it often happens that the first branchial arch 

 is better developed than is the hyoid arch. In the Crocodilia 

 and Chelonia there is a large basilingual plate or body of the 

 hyoid (fig. 53, 1) ; but while in the Crocodilia the first branchial 

 forms the only well-developed arch, in the Chelonia the first 

 and second branchial s are both strongly developed, and the 

 hyoid is often fairly large. 



