TEETH OF KEPTILES. 385 



afterwards divided and comminuted by the strong maxillary 

 dental plates. 



§ 71. Teeth of Reptiles. — If we compare the dental system 

 of Lepidosiren with that in Batrachia, it is to the larval state 

 of the Anourans that an analogy may be found : the tadpole of 

 the Frop; having its maxilla and mandibula each sheathed with a 

 continuous horny trenchant covering. Were this sheath actually 

 dentinal in tissue and united to the jaw-bone, the resemblance to 

 the Lepidosiren would be closer : but it is never calcified, and is 

 shed during the progress of the metamorphosis. 1 The Siren 

 alone, among the perennibranchiates, retains the sheath upon 

 the extremity of the upper and lower jaws ; it consists of a firm 

 albuminous tissue, and becomes harder than horn. But these 

 trenchant mandibles, which play upon one another like the blades 

 of a pair of curved scissors, are associated with numerous small 

 but distinct true teeth, which are grouped together to form a 

 rasp-like surface on each half of the divided vomer, and which 

 beset the alveolar border of the splenial element of the mandible 

 below. 



The whole order of Chelonia is edentulous, as well as the family 

 of Toads (Bufonidaz) in the order Batrachia ; certain extinct 

 genera of Saurians were likewise edentulous, e. g. Rhynchosaurus 

 and Oudenodon? 



In the Tortoises and Turtles the jaws are covered by a sheath 

 of horn, which in some species is very dense ; its working surface 

 is trenchant in the carnivorous species, but is thick, variously 

 sculptured, and adapted for both cutting and bruising in the 

 vegetable feeders. The developement of the continuous horny 

 maxillary sheath commences, as in the Parrot tribe, from a 

 series of distinct papillae, which sink into alveolar cavities, 

 regularly arranged (in Trionyx) along the margins of the upper 

 and lower jaw-bones : these alveoli are indicated by the persist- 

 ence of vascular canals long after the originally separate tooth- 

 like cones have become confluent, and the horny sheath com- 

 pleted. 



The teeth of the dentigerous Saurian. Ophidian, and Batrachian 

 Reptiles are, for the most part, simple and adapted for seizing 

 and holding, but not for dividing or masticating, their food. The 

 Siren alone combines true teeth with a horny maxillary trenchant 

 sheath, like that of the Chelonian Reptiles. 



1 The large dental plates of Lepidosiren have their nearest homologues in those of 

 the extinct fish called Ceratodus (v. pi. 22, fig. 2). 



2 ccxxin. p. 54, pi. I. fig. 1. 



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