PHYLOGENY. 



145 



tures ; for, the earliest dentitions are the most simple, 

 and the later the more complex. Some of the types 

 retain the primitive tritubercular molars, as the Cente- 

 tidae, shrews, and some lemurs, and most Carnivora 

 (above), but the quadritubercular and its derivative 

 forms are by far the most common type in the recent 

 fauna. The forms that produced the complicated mod- 

 ifications in the Proboscidia and Diplarthra appeared 

 latest in time, and the most complex genera, Elephas, 

 Bos, and Equus, the latest of all. The extreme sec- 

 torial modifications of the tritubercular type, as seen 

 in the Hyaenidae and the Felidae, are the latest of their 

 line also. 



Some cases of degeneracy are, however, apparent in 

 the monodelphous Mammalia. The loss of pelvis and 

 posterior limbs in the two mutilate orders is clearly a 

 degenerate character, since there can be no doubt that 

 they have descended from forms with those parts of 

 the skeleton present. The reduction of flexibility seen 

 in the limbs of the Sirenia and the loss of this charac- 

 ter in the fore limbs of the Cetacea are features of de- 

 generacy for the same reason. The teeth in both or- 

 ders have undergone degenerate evolution ; in the later 

 and existing forms of the Cetacea even to extinction. 



The Edentata have undergone degeneration. This 

 is chiefly apparent in the teeth, which are deprived of 

 enamel, and which are wanting from the premaxillary 

 bone. A suborder of the Bunotheria, the Tillodonta 

 of the Lower Eocene period, display a great reduc- 

 tion of enamel on the molar teeth, so that in much- 

 worn examples it appears to be wanting. Its place is 

 taken by an extensive coat of cementum, as is seen in 

 Edentata, and the roots of the teeth are often undi- 

 vided as in that order. 



