T 



/ 



252 



M0N001{AI'liS OF NOUTII AMERICAN KODENTIA. 



SfW:;:;- 



proininnnt aiij^ular iintitrngus. Whiskers sliort. Pelnge laniiginous, beset with 

 mimeroiis long, glossy hairs. Mainiiiit' six. Highly developed jierina'al glands. 

 13pntition strictly arvicoiiiie in every respect ;* skull thoroughly arvicoline; 

 hut sijuuiiiosuls 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 rm)t of 

 the zygoma. An angular ])roces8 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 Marina, 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 lirst 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 jj-, V-i 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 plnue, 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. 



* Aiulubon (i. 107) iioticps some siiigulur orront antbors bavo couiinitt^il in deBcribiug tbe duntltiou ; 



Illigcr, GriQltb, VViegmuuu, unil Kuthu aiwiguiug tbo uiulura 



4.j 

 4-4" 



