252 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



prominent angular antitragus. Whiskers short. Pelage lanuginous, besel with 

 numerous long, glossy hairs. Mammae six. Highly developed perinasal glands. 

 Dentition strictly arvicoline in every respect;* skull thoroughly arvicoline ; 

 bul squamosals greatly expanded, with corresponding reduction of parietals 



and interparietal ; interorbital constriction of frontal at a maximum ; anterior 

 border of outer wall of anteorbital foramen wholly underneath the root of 

 the zygoma. An angular process of squamosal overhanging orbit behind ; 

 zygomatic spur of squamosal touching zygomatic process of maxillary; jugal 

 a mere splint applied internally. 



Fiber is a true arvicoline, showing every essential character of the sub- 

 family as distinguished from Marince, and presenting no features of more than 

 generic grade. Its cranial and dental characteristics depart but little, and 

 only in superficial respects of mere contour; while its more considerable 

 external modifications relate entirely to the highly aquatic habits of the 

 animal. In the upper jaw, the first molar has an anterior triangle, two 

 interior and two exterior triangles, alternating, the first interior following the 

 anterior one. The second molar has an anterior, an interior, and two exterior 

 triangles, alternating, the first exterior following the anterior one. The back 

 molar has an anterior, then an exterior, then an interior triangle, finishing 

 with a simple posterior [}-, \J-, or Y-shaped ti-efrle. In the lower jaw, the 

 first molar, which is wider than, and nearly as long as, the other two together, 

 consists of an anterior treffle, three exterior and four interior triangles, and a 

 posterior loop across the tooth ; but the anterior pair of these lateral triangles 

 do not always close up, so that they frequently resemble mere lobes of the 

 anterior treffle, leaving but two exterior and three interior perfectly closed 

 triangles. The middle molar consists of two exterior and two interior alter- 

 nating closed triangles and a posterior transverse loop. The back molar 

 repeats the middle one, but is still smaller, and the first (antero-exterior) 

 triangle may be a mere spur, or obsolete. The upper incisors describe an 

 almost perfect semicircle in the jaw ; their face is plane, very oblique; they 

 are deeply beveled behind by attrition with the under incisors; these 

 traverse the lower jaw to the root of the condylar process. The jaw is 

 massive; the coronoid is on a level with, or overlaps, the condyle; the 

 descending process is hamular, as usual, and much twisted. 



* Audubon (i. 107) notices some singular errors authors have committed in describing the dentition; 

 Illiger, Griffith, Wiegmanu, and Kuthe assigning the molars — . 



