TEETH. 



215 



Reptilia. Corresponding with the great firmness and solidity 

 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, Skinks, Amphisbsenians, and 

 others), or they lie on the free upper border of the jaw (acrodont 

 dentition), or finally, as in Crocodiles and numerous fossil Reptiles, 

 they are lodged in alveoli (thecodont dentition) (comp. Fig. 175, 

 A, a, b, c). Both upper and lower jaws, and occasionally the 

 palate also, are toothed ; the teeth have a single apex, except in 

 Lizards, in which the apex is double. 1 In many Reptiles, however 

 (e.g. Hatteria, Uromastix spinipes, Agamse, and numerous fossil 

 forms, especially those of the Trias of South Africa), a heterodont 

 dentition, consisting of incisor- canine- and molar-like teeth, is 

 already seen. 



FIG. 175. A, DIAGRAMS OF TRANSVERSE SECTIONS THROUGH THE JAWS OF REPTILES, 

 SHOWING PLEURODONT (a), ACRODONT (&), AND THECODONT (c) DENTITIONS. 

 B, a, LOWER JAW OF Zootoca vivipara ; b, OF Anguis fragilis. (After Ley dig. ) 



The dentition of poisonous snakes deserves special 

 attention, for in them a varying number of maxillary tee'th are 

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

 (Pelias berus and P. prester) there are on each side nine poison- 

 fangs arranged in transverse rows ; the stronger ones project freely, 

 while the lesser, reserve teeth, lie within the gum (Fig. 176, A) ; 

 only one of these teeth, however, is firmly fixed to the maxilla at a 

 time. Each fang is perforated by a poison-canal, which is in- 

 completely surrounded by the pulp-cavity, the latter having the 

 form of a half-ring in transverse section (Fig. 176, B, C, GC, PH) : 

 the duct of the poison-gland passes into an aperture at the base 

 of the tooth which leads into the poison-canal, and the latter opens 



1 A peculiar tooth is present in the embryos of Lizards, Blindworms, and some 

 Snakes. It projects considerably beyond its neighbours, and lies in the median line 

 of the lower jaw extending vertically towards the snout, and serving the young as a 

 means of breaking through the egg-shell. 



