﻿72 



NORTH AMERICAN FAUNA. 



[No. 12. 



Fig. 37. 



-Skull of Fiber zibethicus 

 (natural size). 



^kuU. — The skull (tig. 37) differs very sliohtly from that of Mierotiis 

 except that it is considerably larger than in any known species of the 



latter, and has a proportionally longer ros- 

 trum. The bony palate (PI. II, fig. 12) re- 

 sembles that of the species of Aliicola and 

 J^eofiher in the extension forward of the in- 

 terpterygoid fossa and suppression of the 

 sloping part of the median ridge. The pos- 

 terior border is thus squarely cut off imme- 

 diately behind the lateral bridges. A vestige 

 of the sloping ridge usually persists in the 

 form of a median spine projecting into the 

 iiiterpterygoid space. The skull of Fiber is 

 peculiar in the expansion of the squamosals 

 on the dorsal surface of the skull at the ex- 

 pense of the parietals. The i)ostorbital i^roc- 

 esses of the squamosals form ]Droniinent 

 triangular projections closely resembling 

 those of JS^eofiher. The interparietal is 

 squarish iu outline and usually somewhat 

 longer transversely than antero-posteriorly. 



Teeth. — The molars are all rooted in the adults (fig. 38), though the 

 roots on the back lower tooth are usually 

 less well developed than those on the 

 others. Otherwise the teeth are exactly 

 as in MicroUis. The enamel pattern 

 (fig. 39), most closely resembles that of 

 Microtus {Neofiber) (dJeni, but differs in 

 the larger anterior loop of the first lower 

 molar. This loop is cut by two deep 

 reentrant angles, which often isolate 

 two additional closed triangles, making 

 seven in all. 



Feet. — The feet are large and so formed 

 that they can be turned edgewise when carried forward, thus producing 

 the least possible resistance to the water while the animal is swimming. 



This character is, however, to a certain ex- 

 tent, reproduced in the more aquatic species 

 of Microtus and can not be considered diag- 

 nostic of Fiber. 



Miscellaneous characters. — The tail is strong- 

 ly compressed laterally, making an effective 

 rudder. The peculiar form of the tail is 

 scarcel}^ noticeable in the young even when 

 large enough to leave the nest, but develops 

 rapidly as the animals increase in size. 



The fur of the species of Fiber is highly modified to produce a 



]8.— bide view of molars, Fiber zibeth- 

 icus. (xU.) 



Fig. 39.— Enamel pattern of molar 

 teeth, Fiber zibethicus. (x 24.) 



