TEETH 243 



agjain, be delicate and bristle-like (Chsetodon), or sabre-shaped 

 (Chauliodus). 



In the Dipnoi (Fig. 62) the teeth are compound and exceedingly 

 massive, presenting sharp edges and points. 



In the Amphibia there is in general a considerable diminution 

 in the number of teeth as compared with Fishes ; and at the same 

 time a much more uniform character is noticeable in their form 

 throughout (Fig. 193, A, B). They are enlarged conically below, and 

 rest on a definite base, while above they become narrower and 

 slightly curved, ending either in a double (Myctodera, Anura), or 

 a single apex (Perennibranchiata, Derotremata, Gymnophiona); the 

 latter is the more primitive condition. The teeth lie deeply em- 

 bedded in the mucous membrane, and are present, as a rule, on the 

 preinaxiila, maxilla, and mandible (except in Anura), as well as on 

 the vomer and palatine, but rarely on the parasphenoid (certain 

 TJrodeles, Fig. 192) ; in the larvae of Salamanders and in Proteus the 

 splenial of the lower jaw is also toothed. Horny teeth and horny 

 jciu's, developed entirely from the epidermis, are present in larval 

 Anura, and similar structures occur in Siren lacertina. 



Teeth are altogether absent in the Bufonidse and in Pipa. 



The teeth of certain of the Stegocephala (Labyrinthodonta) 

 were extremely complicated, the enamel appearing as numerous 

 corrugated folds extending from the periphery towards the centre 

 (Fig. 193, C). 



Reptiles and Birds. Corresponding with the greater firm- 

 ness of the skull in Reptiles, the dentition is usually strongly 

 developed, and occasionally at the same time it is more highly 

 differentiated than in Amphibians. The teeth are either situated 

 upon a ledge on the inner side of the lower jaw, with which they 

 become fused basally (pleurodont dentition most Lacertilia) ; or 

 they lie on the free upper border of the jaw (acrodont dentition 

 Chameleon) ; or finally ,as in Crocodiles and numerous fossil Reptiles, 

 they are lodged in alveoli (thecodont dentition) (Fig. 194, A, a, b, c). 

 Both upper and lower jaws, and occasionally the palatine and 

 pterygoid also, are toothed (Lizards and Snakes) ; and in Hatteria, 

 vornerine teeth may also be present. The teeth are usually conical 

 and more or less pointed, but in Lizards the apex is double, and in 

 many Reptiles (e.g., Palaeohatteria, Hatteria, Uromastix spinipes, 

 Agama?, and numerous fossil forms, especially the Theriodontia of 

 the Trias of South Africa), a heterodont dentition is already indi- 

 cated. Almost all Reptiles are polyphyodont. 



In poisonous Snakes a varying number of maxillary teeth are 

 differentiated to form poison-fangs. Thus in the common Viper 

 (Pelias berus) there are on each side ten poison- fangs arranged 

 in transverse rows ; the stronger ones project freely, while the lesser, 

 reserve teeth lie within the gum (Fig. 195, A) ; only one of these 

 teeth, however, is firmlv fixed to the maxilla at a time. Each fang 



K 2 



