THE INSECTIVORES 103 



of the mandible gives the skull a superficial resemblance to 

 the Marsupials which however is not confirmed upon fur- 

 ther study. The canines are reduced and the last premolar in 

 each jaw is completely molariforni. The last molar is greatly 

 reduced as would be expected in so specialized an animal and 

 in each jaw the first molar is the largest of the three. The 

 upper molars are triangular, compressed from before back- 

 ward and elongated from side to side. There is a single in- 

 ternal cusp flanked on its palatal side by a well-marked 

 cingulum. Laterally placed are two cusps, the homologies of 

 which are obscure. The lower molars also are triangular, 

 having on the labial aspect the apex formed by a tall cusp, the 

 protoconid. On the lingual side are two cusps, paraconid and 

 metaconid which are related to the protoconid almost as in 

 typical Jurassic Trituberculates. On the distal or posterior 

 face of the tooth is a small spur-like heel. The lower molars 

 fit between the upper teeth and thus form well-marked shears. 



The inference to be drawn from examination of the teeth 

 of the Zalambdodonts is that they must be considered apart 

 from more truly tritubercular tuberculo-sectorial dentitions 

 exhibited by other Mammals. The Marsupial Mole either is 

 derived with the Zalambdodonts from a remote common an- 

 cestor or has attained the very specialized features of its denti- 

 tion by convergence. 



Gregory's most recent attempt to homologize the cusps of 

 the teeth in Chrysochloris depends upon a modified conception 

 of the Premolar Analogy Theory. Suffice it to observe that 

 the first permanent molar quite possibly belongs in reality to 

 the milk dentition, whereas the permanent premolars belong 

 to the successional set. Looked at from this standpoint pre- 

 cise homologies of the cusps of the permanent molars must not 

 be sought directly in the premolars but in the milk molars 

 (see Fig. 77). It is known that the first cusp to develop in the 

 upper molar is the paracone and that the protocone develops 

 later: hence it is probable that the paracone is phylogenetically 

 the older cusp and that it represents the apex of the primi- 



