REPTILIA. 387 



The Chelonia, the Cretaceous genus Pteranodon among Pterodactyls, 

 and Oudenodon among the Dicynodontia, are edentulous 1 . The fore-part of 

 both jaws is devoid of teeth in some Deinosauria, e. g. Iguanodon, and in 

 some species of Rhamphorhynchus among Pterodactyls. A thickened epi- 

 dermic sheath probably covered these parts as in the living Chelonia. 

 Teeth are generally confined to the praemaxilla, maxilla and dentary, but 

 in the Ophidia and some Lacertilia the palatine and pterygoid are den- 

 tigerous also. They are anchylosed either to the edge of the jaw, e.g. 

 Ophidia, many Lacertilia, or by their sides to a groove, e. g. Iguana ; or are 

 lodged in alveoli, e. g. Crocodilia, many Deinosauria ; or in a shallow groove 

 scarcely divided into alveoli as in Ichthyosauria. These forms of dentition 

 are known respectively as acrodont, pleurodont, and thecodont. They are 

 ranged usually in a single row, but there are exceptions as in some 

 Ophidia. Anchylosis takes place by true bone, and, in Hatteria, when the 

 teeth are worn away, the dense bone at the edge of the jaws forms an 

 efficient substitute. The enamel is often ridged and sometimes plicated at 

 the base of the tooth in Lacertilia. Succession is vertical in Crocodilia, 

 many Deinosauria ; or the new tooth may take a lateral position as in the 

 poison fangs of Ophidians. The two tusks of the extinct Dicynodon grew 

 from permanent pulps, whilst the rest of the jaw was probably covered by 

 thickened epidermis. The form of .the teeth is very variable. The extinct 

 Theriodontia have teeth resembling incisors, canines, and molars. Grooved 

 teeth occur in Ophidia, and in poisonous serpents the groove is well- 

 developed in one tooth on each side and utilised to convey the secretion of 

 the poison gland into the wound made by it. Somewhat similar but less 

 specialised teeth occur in the poisonous lizard, Heloderma (Nature, xxvii. 

 1883-83: xxviii. 1883). A single large tooth implanted on the prae- 

 maxillae serves to rupture the egg-shell in Lacerta and Anguis (Lacertilia) 

 and in Tropidonotus (Ophidia]. Salivary glands are absent in marine 

 Chelonia, and the Alligator : they are confined to the tongue in the 

 Crocodile, but, as a rule, there are not only lingual glands, but also sub- 

 lingual, palatal, and labial. A differentiation of one of the upper labials 

 forms the poison gland of Ophidia. The tongue may be spatula-shaped 

 and immobile, as in Chelonia and Crocodilia and some Lacertilia, or bifid, 

 protrusible and more or less elongated as in other Reptilia. The oeso- 

 phagus is wide and sometimes, as in Ophidia, not clearly marked off 

 from the stomach. In marine Chelonia there are large papillae with 

 points directed backwards towards the stomach. The cardiac portion 

 of the latter is enlarged in the Crocodilia and shows on its flattened 

 dorsal and ventral surfaces a tendinous spot whence the muscle fibres 

 radiate ; it is followed by a small pyloric enlargement. The small intes- 

 tine is disposed in numerous coils in the Chelonia and Crocodilia. In the 



1 Trwnyx (Chelonian) is stated by Wiedersheim to have embryonic teeth. 



C C 2 



