92 MICROTINvS; 



2. External characters highly modified for aquatic habits; 

 under fur very dense and fine, contour hairs long, 

 stiff and glossy; palms and soles naked; hands and 

 feet furnished with " swimming-fringes " ; claws 

 enlarged. Mammte 1—2=6. Size very large. 



Skull with the temporal ridges fused in the inter- 

 orbital region when adult. Auditory bullae small ; 

 external meatus tubular; stapedial artery enclosed 

 in a bony tube as far as stapes. 



Dentinal spaces of cheek-teeth always closed; m^ 

 with closed triangles; re-entrant folds partly filled 

 with cement. 



a. Cheek-teeth rooted in adults; m^ with three or four 



salient angles on each side ; mj with five outer and 

 six inner saKent angles and from five to seven 

 closed triangles; wij with three outer sahent 

 angles. 



Skull with temporal ridges closely approximated 

 throughout ; its dorsal contour flattened from before 

 backwards ; post-orbital processes of squamosals 

 forming shelf-like crests. 



Tail laterally compressed and scaly. Feet 

 obviously twisted; lateral margins of toes con- 

 spicuously fringed ; fore and hind claws subequal. 



Contour hairs not forming a " dorsal keel " on 

 rump Ondatra. 



b. Cheek-teeth rootless and persistently growing; m^ 



with three sahent angles on each side ; mj with four 

 outer and five inner salient angles and five closed 

 triangles; JWg with two outer salient angles only. 

 Skull with temporal ridges -widely separated on 

 the braincase ; its dorsal contour convex from before 

 backwards; post-orbital processes of squamosals 

 prominent and peg-like. 



Tail terete, densely haired. Feet less noticeably 

 twisted; "swimming-fringes" less developed on 

 borders of digits; hind-claws longer and stouter 

 than fore-claws. 



Contour hairs forming a " dorsal keel " in neigh- 

 bourhood of rump Neofiber. 



B. Skull with palate not terminating behind as a transverse 

 sheK; median spinous process always present, con- 

 verted into a sloping septum between the postero- 

 lateral palatal pits, the inner borders of these pits 

 always continuous with the tip or sides of the median 

 process. Auditory bullse with or without spongy tissue. 

 1. Cheek-teeth rooted in adults. Bony palate with 

 postero-median sloping septum short and broad. 

 a. Skull with the temporal ridges fused in the interorbital 

 region when adult ; palate with postero-lateral pits 

 shallow and ill defined, the postero-lateral bridges 

 sometimes absent. Auditory bulla} large with thin 

 walls; mastoid portion shghtly inflated; stapedial 

 artery enclosed in bony tube as far as stapes. 



Mandible with wij not noticeably cUsplaced by 

 shaft of incisor. 



