KINETOGENESIS. 333 



simplicity for a much longer period, a simplicity which 

 they retain in the Carnivora, except the J-, which be- 

 came the sectorial. Secondly, the premolars, instead 

 of increasing in size, have in many types decreased ; 

 the Diplarthra alone presenting an exception to this 

 rule. That the internal cusps of the premolars may 

 have arisen by growth of cingula in this order, is by no 

 means improbable. We seem to have here an excel- 

 lent illustration of the origin of two identical struc- 

 tures by different evolutionary routes. 



The first modification of the tritubercular molar of 

 the lower jaw is the addition of a low cingulum at the 

 posterior base. This is seen in a rudimentary condi- 

 tion in various living species of the Centetidse and 

 Chrysochloridse of the insectivorous order (Fig. 100); 

 but in these existing forms the superior molar has 

 added a posterior cingulum also, which widens inter- 

 nally, or towards the palate (Fig. loi). In the evolu- 

 tion of the dentition, the inferior posterior cingulum, 

 or "heel," was developed first, as in the Deltatherium, 

 Centetes, and Stypolophus (Figs. 99, 100, 102), where 

 it is quite large ; while the superior cingulum is want- 

 ing in Stypolophus and Didelphodus, but is present in 

 a very rudimentary condition in Deltatherium fundami- 

 nis. In all of these genera the external cusps of the 

 superior series have been pressed inwards, and more 

 or less together, and are therefore removed in this re- 

 spect from the primitive condition. The more primi- 

 tive state of the superior cusps is seen in some species 

 of Mioclsenus, where, however, a posterior cingulum 

 may be developed. The primitive type of tritubercular 

 superior molar is that of Sarcothraustes, and in the 

 same genus the inferior molar only differs from the 

 primitive type in having a well-developed heel. Among 



