MAMMALIA 337 



tend to become tuberculate (bunodont) in some groups, and flatten 

 out into broad, crushing teeth (lophodont) in others. In some orders 

 the incisors are modified into great chisel-edged gnawing teeth; in 

 other groups they become vestigial or are totally lost in the adult. 

 The canines on the whole are the most conservative of the teeth, tend- 

 ing to retain their conical shape; but in some groups they have be- 

 come specialized into tusks of various kinds, and in other groups they 

 are vestigial or absent. 



The feet of primitive reptiles are typical five-fingered feet with 

 claws. From this type of foot, it will be recalled, the reptiles under- 

 went an extensive adaptive specialization. Hoofed feet were de- 

 veloped in the heavy herbivorous forms and long raptorial claws de- 

 veloped among the carnivorous groups. A similar adaptive radiation 

 in foot structure occurred among the mammals. It is probable that 

 the first mammals were unguiculate (clawed), a condition very similar 

 to the generalized ancestral foot. From this type were developed 

 the various three-fingered, two-fingered, and one-fingered hoofed 

 types, the curving-clawed preda- 

 ceous types, the flat-nailed types of 

 the primates, and all of the other 

 specialized types of foot structure. 



The earliest mammalian remains ,-, 

 /-n- IT/IX > e > i * IG - ** Jaw of primitive mam- 



,^lg. 17$) consist Ot two jaw bones m^ Dromatherium sylvestre, Trias, 



found in the coal beds of North N. Carolina; twice natural size. 

 Carolina, a Triassic deposit. The (From Lull, after Osborn.) 

 creatures to which these jaws belonged, whether they were true mam- 

 mals or not, must therefore have been contemporaneous with the 

 South African cynodonts. Except for the fact that the jaw was a 

 one-piece jaw consisting only of the paired dentary bones, it was 

 more like those of the cynodonts than like those of modern mam- 

 mals. The molar teeth were very generalized in that their num- 

 ber of tubercles was indefinite and the incisors were only slightly 

 flattened. 



A somewhat later group of primitive mammals, known as Tricono- 

 donta, is represented by a few fragmentary remains (Fig. 175) found 

 in Jurassic rocks. These mammals had teeth more perfect in form 

 than those just described, the molars being trituberculate with the 

 cusps knife-edged and arranged in a single row like the teeth of a saw. 

 Another group of early mammals of Lower Cretaceous and Jurassic 



