EDWARD D. COPE. 77 



three-sided prism, with tritubercular apex, which alter- 

 nates with that of the opposite jaw. Fourth, develop- 

 ment of a heel projecting from the posterior base of the 

 lower jaw, which meets the crown of the superior, form- 

 ing a tubercular-sectorial inferior molar. From this stage 

 the carnivorous and sectorial dentition is derived., the 

 tritubercular type being retained. Fifth, the develop- 

 ment of a posterior inner cusp of the superior molar and 

 the elevation of the heel of 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 produced 

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

 various 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 oldest (Eocene and Miocene) 

 types are but two-rooted and compressed, having much 

 the form of the premolars of other Mammalia. In exist- 

 ing 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 

 compared with those of their extinct ancestors displays 

 a great deal of improvement in many ways, and but few 

 instances of retrogression. The succession in time of the 

 Monotremata, the Marsupialia, 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 themselves, it is 

 more than probable that the order has degenerated in 



61 



