352 Wo7'tma7i — Studies of Eocene Mammalia in the 



in whicli the cheek teeth had ahiiost completely disappeared. 

 We have no information of the structure of the hand; but 

 whether or not any of the fingers were modified in a manner 

 corresponding to that of the Aye Aye, the practical loss of the 

 molars and premolars can be accounted for on no other supposi- 

 tion than that the nature of the food upon which the animal 

 subsisted was so soft as to require no crushing power on the 

 part of the grinders. From what we know of the habits of 

 the Aye Aye, the inference is both logical and natural that 

 this food was also soft larvae, which the animal was doubtless 

 accustomed to seek in a similar way. From the comparative 

 slenderness and weakness of the lower jaws, we may even 

 further suggest that the animal captured these grubs in soft or 

 decayed wood. 



If the facts of structure have been correctly interpreted and 

 our hypothesis in i*egard to the habits is well founded, what 

 shall we say of the relationship between Metacheiromys and 

 CheiTomys f Is it possible to suppose that these modifications, 

 so profound and unique among the Primates, have originated 

 twice in the same group entirely independently of each other? 

 Metacheiromys can not be placed directly in the ancestral line 

 of Cheiromys for the reason that by the loss of the grinders it 

 had, in the Eocene, already reached a more advanced stage of 

 evolution than the living genus. But to deny that the two 

 were descended from a common ancestral stock would, it 

 seems to me, involve such a tremendous assumption as to lay 

 a heavy burden upon our powers of belief. Such assumption 

 becomes all the more onerous in the complete absence of any 

 evidence in its support. If one had no proofs upon which to 

 base an opinion respecting the community of origin and dis- 

 tribution from a common center other than that afforded by 

 these two animals, so widely separated in space and yet so 

 closely connected in structure, he could still feel amply assured 

 of the security of his foundations. This evidence of the 

 relationship between the Madagascar and Wyoming species, 

 therefore, adds but another link in the chain of proof already 

 set forth, that both forms were migrants from a common 

 boreal home. 



Family Microsyopsidoe. 



Microsyops Leidy. 



The next family of this group to be considered is the Micro- 



syopsidse. The type genus Microsyojps was separated and 



described by Leidy in April, 1872.* In June, 1871, Marsh 



had previously described a species, Hyopsodus gracilis,-\ which 



*Proc. Acad. Nat. Sci. Phila., 1873, p. 20 (published April 16). 

 f This Journal, vol. ii, 1871, p. 42. 



