1,3 • 



CHORX)ATE ANATOMY 



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

 tion, some lizards 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, limit their teeth to the two jaws. 



An especially elongated tooth occurs in the lower jaw of lizard 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. 



A C 



Fig. 123. — Jaws of some primitive Jurassic mammals. The resemblance of these 

 jaws and teeth to those of the theriodont reptiles of the same period suggests a similar 

 genetic origin. (Redrawn from Romer, after Simpson.) 



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 pairs 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 acquired also a 

 definite succession of sets of teeth, a set of "milk teeth" developed in 

 early life being later replaced by a "permanent" set. 



The elasmobranchs, indeed, do have a certain succession of teeth, 

 but only one at a time as single teeth are lost. Lower vertebrates, reptiles 

 conspicuously, have a somewhat indefinite number of sets, and are said 

 therefore to be polyphyodont. Only the mammals have two definite 



