202 ANIMAL LIFE PAST AND PRESENT. 



crushing the shells of crustaceans and molluscs. Pass- 

 ing these reptilian teeth, interesting as they are, with 

 this cursory notice, we turn to those of mammals. 



Starting from a tooth of the simple conical type of 

 the one represented in Fig. 61, it is possible to evolve 

 therefrom the whole of the curious modifications of 

 tooth-structure found in the higher Vertebrates. The 

 first stage in the process is the development of small 

 points, or cusps, on either side of such a cone, as is 

 seen in the hinder teeth of the skull of the extinct 

 reptile shown in Fig. 61. A further step is for these 

 side cusps to become nearly or quite as large as the 

 original main cone (Fig. 63, A), and then to be twisted 

 round upon the same, so that in the upper jaw 



(as is shown in the hinder 

 teeth of the Civet, figured in 

 woodcut 64), each upper tooth 

 has one inner and two outer 

 cusps, while exactly the re- 

 verse arrangement obtains 



lG. 63. Teeth of Secondary Mam- . 



mala, enlarged. A, Side view 111 the teeth OI the lower 



of lower tooth of a species with ,~. -., mi 11- 



the three cusps in the same 4ine. J clW (**% > A)' 



B, Do. of a lower tooth of a species t | on Q f ex t ra CO116S Or CUSDS 

 in which the cusps form a triangle. 



l.r, outer cusp ; pa, anterior do. ; to this triangular form of 

 nte, posterior do. i i 



tooth, with alterations or the 



simple cusps into ridges, either by union with one 

 another, or by flattening in one direction or another, or 

 by increase in their height, will produce all the com- 

 plicated types of hinder teeth found in the higher 

 Mammals. 



