214 MAMMALIAN DENTITION 



third milk incisors, canines and all the milk molars are still 

 in place. 



One of the most interesting and significant features of the 

 Tapir dentition is the occurrence, according to the late Dr. 

 Ameghino, of at least one functional premilk tooth in the 

 premolar region. To this we shall refer later in the chapter 

 upon the deciduous dentition. The low-crowned molars of 

 both jaws show the formation of transverse crests or lophs 

 through the union of anterior and posterior pairs of cusps, 

 a conformation of occlusal surface evolving in browsing 

 species of many mammalian orders. The elongated or hypso- 

 dont character of the loph associated with the brachyodont 

 bodies of the cheek teeth is very noticeable ; it recalls the 

 similar arrangement in the Kangaroos. The four cusps pre- 

 sented by each of the upper teeth are those of the trigon 

 with the addition of the hypocone and in the lower teeth the 

 proto-, meta-, hypo-, and ento-coiiids, the paraconid as usual 

 being lost. Well-marked anterior and posterior cingula are 

 to be seen on all molars. There is also a poorly marked ex- 

 ternal ridge or ectoloph connecting paracone and metacone of 

 the upper molars. 



The Rhinoceroses form one of the largest and most compli- 

 cated groups of Mammals provided one include all extinct 

 species. No less than seven phyla flourished at various times 

 and of these three exist today. In addition there were two 

 other series allied to the Rhinoceroses in the Eocene and Oli- 

 gocene periods, one a lightly built cursorial form, the other 

 heavy and aquatic. 



So far as teeth are concerned the modern type was reached 

 in the Pliocene. All Ehinoceroses have low-crowned browsing 

 cheek teeth except the White or Square-lipped Rhinoceros of 

 Africa, a grazing variety with fairly hypsodont molars which 

 has lost all its incisors and canines above and below. The 

 dental formula is: 



