1 86 COMPARATIVE ANATOMY 



In teleosts, the number of teeth is somewhat reduced, although all 

 parts of the mouth and even the pharynx may carry them. The special 

 advance made by the teleosts is to set the teeth more firmly by fusing 

 their bases with the membrane bones of the mouth. 



Amphibia still farther reduce the number of teeth ; but retain them on 

 premaxilla, maxilla, mandible, vomer, and palatine bones, and more 

 rarely on the parasphenoid. But toads have no teeth whatever. A strik- 

 ing feature of certain ancient and long extinct amphibians, the labyrin- 

 thodonts, which arose in the Coal Period and survived into the Triassic, 

 was the enormously comphcated folding of the tooth enamel and dentine, 

 which anticipated, yet went far beyond the similar arrangement in some 

 mammals. 



Reptiles make two important advances toward the condition in 

 mammals. Some of them, like some of the amphibia, have their teeth 

 set on a ledge on the inner side of the jaw — pleurodont dentition. Or they 

 may have the tooth set directly on the bone, acrodont dentition. But the 

 crocodiles and some fossil reptiles attain to a thecodont dentition, in 

 which each tooth is fixed in a separate socket, as in mammals. In addi- 

 tion, some hzards and numerous fossil reptiles abandon the original 

 homodont dentition, with all teeth about alike, and have their teeth 

 more or less differentiated into incisors, canines, and molars, as in mam- 

 mals, a heterodont dentition. The differentiation of teeth is obviously 

 an adaptive division of labor, the incisors acting as cutters or chisels, 

 the canines as daggers, and the molars as grinders. The reptiles, more- 

 over, hmit their teeth to the two jaws. 



An especially elongated tooth occurs in the lower jaw of Hzard and 

 snake embryos, which is used to break through the tough membranous 

 shell. A hardened tip of the horny beak of birds is used for the same pur- 

 pose. The two structures are, however, morphologically quite different. 



Especially remarkable in reptiles are the highly specialized poison 

 fangs of certain snakes. These are modified from the ordinary conical 

 tooth, first by a folding of the tooth to form a groove along which the 

 venom from a modified salivary gland flows into the wound. In other 

 snakes, by a still further folding the edges of the groove unite, and the 

 tooth becomes a hollow needle. One pair only is functional at any one 

 time, others up to nearly a dozen pair being held in reserve to take the 

 place of the large fangs when these are lost. All the vipers including the 

 rattlesnakes fold back the functional pair of fangs when the mouth is 

 closed, and only in the act of striking pull them erect by special muscles. 

 Mammals, besides having nearly always a heterodont dentition, with 

 incisors, canines, and molars well differentiated, have adopted also a 

 distinction between a milk and a permanent set, as a means of adjusting 

 to an enlarging mouth solid teeth of limited size. 



