252 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
prominent angular antitragus. Whiskers short. Pelage lanuginous, beset with 
numerous long, glossy hairs. Mammee six. Highly developed perineal glands. 
Dentition strictly arvicoline in every respect ;* skull thoroughly arvicoline ; 
but 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 Murine, 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 imterior 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 Y-, Y-, or Y-shaped treffle. 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; 
4-4 
Iliger, Griffith, Wiegmann, and Ruthe assigning the molars 
