INTERRELATIONSHIPS OF GENERA 



75 



Ondatra, but the postero-lateral bridges are more developed, 

 though slender and incomplete. The choanse, pterygoid fossse 

 and auditory bullse are as in Ondatra. 



The mandible and incisors are normal ; the shaft of the lower 

 incisor displaces Wg lingually. The cheek-teeth have acquired 

 the power of growing persistently, although in other respects 

 (enamel, tight closure of dentinal spaces, presence of cement in 

 infolds, and general pattern) they closely resemble those of 



Fig. 47. — Neofiber alleni True. 

 Dorsal view of skull (enlarged). 



Ondatra. The enamel pattern of m^, ?rtj, and ni^ is, however, 

 somewhat simpHfied ; 7n^ has only three salient angles on each 

 side ; ?»j has a posterior loop, five closed triangles, and a short 

 anterior loop (which shows more or less ephemeral and vestigial 

 traces of reduced and obsolete elements) ; in m^ the antero- 

 external angle is suppressed, although the triangles following the 

 posterior loop are substantially closed. 



Two genera of extraordinary interest, namely, Protnetheomys 

 and EUobins, remain for discussion. Certain characters, partly 

 primitive, partly progressive, shared in common, show that 

 these two genera are closely related ; but in spite of this close 



