104 MAMMALIAN DENTITION 



tive simple reptilian tooth. In its turn the main cusp of the 

 premolar has the same homology and thus corresponds with 

 the molar paracolic In Chrysochloris the upper molar cusp 

 which occupies the site upon the crown in series with the main 

 cusp of the premolars is the palatal cusp at the apex of the 

 triangle. This should then be the paracone and the cusps at 

 the base of the triangle represent the external styles: the 

 palatal cingulum corresponds to the protocone. This type of 

 tooth dates back almost unchanged to the Oligocene (Apter- 



Fig. 33.— Dentition of West African Water Shrew (Potamogale velox, 9.333-1). 

 The dentition of this animal stands intermediate between those of the more typical 

 Insectivores and that of the very specialized form Chrysochloris. A milk tooth 

 is retained in this specimen among the upper premolars. The shadow falls so densely 

 between the trigonids of the mandibular molars that the heel or talonid can be seen 

 upon the last one only. 



nodus) and it may be that the metacone like the hypocone 

 never formed. Further light upon the derivation of this type 

 of molar is shed however by Palaeoryctes, a very ancient 

 Mammal occurring in the Paleocene. In this the paracone and 

 metacone are very close together as if the latter had just 

 budded off the former. 



Potamogale velox, the West African Water Shrew (Fig. 33), 



