EVOLUTION" OF THE VERTEBRATA. 347 



epitomizes that of the Mouodelph orders. The dentition of the 

 carnivorous forms is creodont ; that of the kangaroos is perisso- 

 dactyle, and that of the wombats is rodent. Other forms repeat 

 the lusectivora. I therefore consider the placental series espe- 

 cially. I have already shown that the greater number of the types 

 of this series have derived the characters of their molar teeth from 

 the stages of the following succession. First, a simple cone or 

 reptilian crown, alternating with that of the other jaw. Second, 

 a cone with lateral denticles. Third, the denticles to the inner 

 side of the crown forming a three-sided prism, with tritubercular 

 apex, which alternates with that of the opposite jaw. Fourth, 

 development of a heel projecting from the posterior base of the 

 lower jaw, which meets the crown of the superior, forming a 

 tubercular-sectorial inferior molar. From this stage the carnivo- 

 rous and sectorial dentition is derived, the tritubercular type being 

 retained. Fifth, the development of a posterior inner cusp in the 

 superior molar, and the elevation of the heel in the inferior molar, 

 with the loss of the anterior inner cusp. Thus the molars become 

 quadritubercular, and opposite. This is the type of many of the 

 Taxeopoda, including the Quadrumana and Insectivora as well as 

 the inferior Diplarthra. The higher Taxeopoda (Hyracoidea) and 

 Diplarthra add various complexities. Thus the tubercles become 

 flattened and then concave, so as to form Vs in the section pro- 

 duced by wearing ; or they are joined by cross-folds, forming vari- 

 ous patterns. In the Proboscidia the latter become multiplied so 

 as to produce numerous cross-crests. 



The dentition of some of the Sirenia is like that of some of 

 the Ungulata, especially of the suilline group, while in others the 

 teeth consist of cylinders. In the Cetacea the molars of the old- 

 est (Eocene and Miocene) types are but two-rooted and com- 

 pressed, having much the form of the premolars of other Mam- 

 malia. In existing forms a few have simple conical teeth, while 

 in a considerable number teeth are entirely wanting. 



A review of the characters of the existing Mammalia as com- 

 pared with those of their extinct ancestors displays a great deal 

 of improvement in many ways, and but few instances of retro- 

 gression. The succession in time of the Monotremata, tlie Mar- 

 supialia, and the Monodelphia, is a succession of advance in all 

 the characters of the soft parts and of the skeleton which define 

 them (see table of classification). As to the monotremes them- 

 selves, it is more than probable that the order has degenerated in 



