TEETH OF VERTEBEATA. 



551 



lu fhe Selacliii they are partly movable, and are arranged in 

 rows of diifereut ages. In most Fishes they retain their superficial 

 position, and, where they are more firmly united, 

 this is effected by their fusing with the bones 

 which carry them. This is the case also in the 

 Amphibia, where the earliest dental structures 

 form their proper bones by fusing together at 

 their bases. In the Reptilia the teeth are formed 

 independently, like the later teeth of the Am- 

 phibia; sometimes they are mere excrescences 

 (pleurodont Lizards) ; sometimes the developing 

 teeth are sunk into their pi-oper bones. In some of 

 the Saurii, the teeth are attached to the edge of 

 the jaw (acrodont Lizards). In the Geckos and 

 Ophidii, and in all Crocodilini, the developing 

 teeth are partly surrounded by the edges of the 

 jaws, and are, therefore, embedded in alveoli. A 

 similar arrangement obtains in the Mammalia. A 

 mass of epithelium grows into the mucous mem- 

 brane of the edge of the jaw, and forms a cap 

 around a papilla, on which the rudiment of the 

 tooth is developed ; as this follicular structure is 

 surrounded by the jaw the tooth is completely 

 differentiated within the jaw, and only breaks 

 through the mucous membrane as it is gradually 

 developed ; the saccule which forms it is nipped 

 off from this mucous membrane. 



The teeth vary very greatly in form, so that 

 there is every intermediate stage between broad plate-like struc- 

 tures, and long, fine, spicular forms ; this variety of character is 

 most common in Fishes. The teeth of the Amphibia are more 

 similar in form ; in the extant members of the group, at any rate, 

 they are generally simply conical, or faintly notched. Among the 

 Reptilia greater differences are seen in the Saurii, and pai-tly too 

 in the Ophidii, in some of which a certain number of teeth are 

 connected with a special poison apparatus. In the CrocodiHni, also, 

 the conical form is the most common; in them the new teeth are 

 always placed below those which are already developed^ and are 

 covered by them. 



Birds have no teeth. But as fossil forms — the Odontomithes 

 (Ichthyornis, Hesperornis) — are known in which the jaw did carry 

 teeth, their absence in extant forms must be regarded as having 

 been acquired within the limits of the class. 



Among Mammals, the individual tooth varies very greatly, so 

 that a single dental apparatus contains various forms of teeth. 

 These, again, have different functions in relation to the food in- 

 gested, and vary greatly in character according to the kind of 

 food; it is in the Delphinoidea only that the lower condition, in 

 which all the teeth are similar, is retained ; in the Balasnoidea the 



Fig. 309. Diagram 

 of the development 

 of teeth. A process 

 of the epithelial 

 layer is sunk into the 

 naucous membrane, 

 and forms an enamel 

 organ (e) over each 

 papilla (p). 



