heterodonti, that is, structured specialization of different 

 teeth. The front teeth are incisor-like, with three cusps in a 

 line; behind these are pointed conical teeth, and behind 

 these, large, rounded, crushing teeth. The presence of sev- 

 eral cusps may be explained as due to the fusion of several 

 tooth buds or subdivision of the original bud. Although 

 many reptiles are described as homodontous, the conical 

 teeth may differ markedly in size in different parts of the 

 jaw margin. 



In the reptile, teeth are borne on a large number of bones, 

 just as in the amphibian or fish: the premaxilla, the maxilla, 

 the vomer, and occasionally the pterygoid and palatine. 

 Among lizards and reptiles in general, there is a wide range 

 of tooth modification for various functions, for example, the 

 sharp conical tooth as opposed to the broad, flat crushing 

 plate. In the living turtles, teeth are lacking, but, in the 

 most primitive turtle known, Jnassochelys, small conical teeth 

 covered the palate and jaw margins. Like the turde, the 

 bird lacks teeth, although the Cretaceous genus Hesperomis 

 had them. 



Mammals 



The general structure of the mammalian tooth has already 

 been described; it is peculiar in the thickness of its enamel 

 cover and in usually having a tapered root, or roots. The 

 main items of interest in comparing mammals are the vari- 

 ations in the shape of the teeth (heterodonti) and the occur- 

 rence of two sets (diphyodonti), milk and permanent. The 

 teeth are specialized as incisors, canines, and as cheek teeth 

 which grade from simple cones to molariform. Some cheek 

 teeth, the premolars, are preceded by milk teeth, but the 

 molars are not. The fact that the molars are not replaced is 

 probably related to their late appearance. 



In terms of their growth, mammalian teeth vary; diphyo- 

 donti for some of the teeth has already been mentioned. 



tooth cells 



jaw bone-ll. 



germinal layer of epidermis 



Figure 8-63. Horny tooth of mouth margin of 20-mm larval frog. 

 (After Peyer, 1937) 



The brachydont type, such as in man, has a limited period 

 of growth during which the tooth is completely formed and 

 the root or roots close. A second type, the hypsodont or 

 high-crowned tooth, is observed in the molars of the horse. 

 This type has a long growth period and both root and crovm 

 are covered by cement, a bony material. The root does not 

 close till late in life. The last type is seen in the incisors of 

 rodents or rabbits and in many other mammals; growth 

 continues through life, and the root remains open. 



There are several theories as to the origin of mammalian 

 teeth. The simplest is that the several tooth types gradually 

 evolved from the ancestral conical tooth. According to the 

 dimere (du meaning two; meros, parts) theory of Bolk, the 

 primitive reptilian tooth, as well as that of some primitive 

 mammals, the triconodonts, had three cusps (see the front 

 teeth of Tupinambis). Two such teeth fused together would 

 produce a typical molariform tooth. One such tooth minus 

 front and back cusps would form a canine tooth. According 

 to this view, some teeth are trimerous or even polymerous, 

 such as the molars of elephants. Another theory, supported 



germinal layer of epidermis 

 .enamel 

 dentine 



B 



Figure 8-62. Vertical section of tooth and jaw in Crypfobronchus, A, 

 and the frog, B. 



T^blood vessel 



pulp cavity lined by odontoblasts 

 Figure 8-64. Section through tooth and jaw of lizard. (After 

 Kendall, 1947) 



TEETH 



251 



