﻿July, 1896.] 



GENUS FIBER. 



71 



closure of the triaugles in this tooth increases with the size of the 

 animals until in such large species as Microtus alleni and the members 

 of the genus Fiber they are always tightly closed. Microtus terrestris, 

 the only species approaching M. alleni in size, has closed triangles in 

 m 3 very often, while in one si)ecimen the tooth is formed exactly as in 

 Neofiber, M. princii^alis Ehoads, another large species, also rather 

 frequently shows closed triangles in this tooth. The incisors, like those 

 of Fiber, are short, broad, and very strong, in this respect reaching the 

 opposite extreme from that attained by ^Aulacomys.^ 



2Iammm. — Apparently the number of mammse in Neojiber has never 

 been stated in prmt. Mr. Outram Bangs writes me, however, that he 

 found four inguinal teats in an adult female Microtus alleni which he 

 took in Brevard County, Fla., during February, 1895. 



Feet. — Soles wholly naked, foot i^ads five, as in Arvicola; claws on 

 hind feet longest. 



Fur. — The fur is modified to meet the requirements of an aquatic life 

 in the same way and to almost the same extent as in the genus Fiber, 

 The under fur is exceedingly thick, woolly, and dense, while the longer 

 hairs are very glossy and lustrous. This condition is suggested in 

 Arvicokij where, however, the modification is not carried so far. 



Miscellaneous characters. — Whether Xeofiber is i)rovided with musk 

 glands like those of the other water rats is at preseut uncertain. Col- 

 lectors have failed to notice them, but they might easily escape detec- 

 tion in the thick fur unless si)ecially searched for. The only alcoholic 

 specimen that I have examined is not full grown. This shows no trace 

 of the glands even when the skin of the sides is raised and examined 

 from beneath. 



General remarlis. — In Neojiber are combined the mandibular enamel 

 pattern of Lagurus with the maxillary enamel pattern and external 

 characters of typical Arvicola, complicated by a reduction in the num- 

 ber of mamm?e as in Fedomys and Fitymys. 



Geuiis FIBER Ciivier. 



Fiber Cuvier [Tabl. ]^]6m. de I'Hist. Nat. des Anim,, p. 141, 1798], Lemons d'Auat. 

 Comp., I, tabl. I, 1800. Type Castor ziiethicus Linn. 



Geographic distribution of type species. — ISTorth America north of the 

 southern border of the United States. 



Geographic distribution of gemis. — The range of the genus Fiber is 

 essentially the same as that given for the type species. 



Fssential characters : 



Upper incisors with anterior faces smooth. 

 Lower incisors with roots on outer side of molars. 

 Molars rooted. 



Enamel pattern characterized by approximate equality of reentrant angles on 

 outer and inner sides of molars. 

 Feet modified for swimming. 

 Tail flattened laterally. 



