6 MICROTIN^ 



than the hind-foot. The skin of the tail is scaly, the scales 

 usually forming rather well-marked annulations; it is more or 

 less well clothed with stiff hairs, which may or may not conceal 

 the scales ; frequently a distinct terminal pencil of variable length 

 and thickness is developed. 



The fur tends to be soft and dense in all the Microtinse; in 

 those most highly speciahzed for fossorial life it is very short, 

 fine, uniform, silky, and mole-like in texture; whereas in the 

 aquatic forms the contrast between the silky, dense under fur 

 and the longer, stifier, and more lustrous contour-hair is intensified. 



The normal mammary formula in the group is 2 — 2 = 8, there 

 being two pectoral and two inguinal pairs of mammse in the females. 

 In many genera one or both pairs of pectoral mammae become 

 functionless and disappear, the formula being reduced to 

 1 — 2 = 6 or 0—2 = 4; exceptionally the inguinal mammae 

 suffer reduction, so that the formulae 2 — = 4, 2 — 1 = 6, 

 and 1 — 1 = 4 are not unknown within the subfamily. 



In many genera specially developed glands are present upon 

 the flanks or hips in adult males and sometimes in both sexes. 

 Special perineal glands, secreting a powerful musk, occur in the 

 genus Ondatra. 



Skull. 



The skull of the Microti nae is always of firm and often of 

 relatively massive construction, the sagittal sutures between the 

 paired frontal, premaxillary, maxillary, and palatine bones 

 generally fusing and disappearing either before or shortly after 

 birth. Normally the skull is characterized by the shortness of the 

 rostrum (in dorsal view), and by the forward position of the orbit, 

 which is always anterior to a vertical plane touching the front 

 edges of the anterior molars. The form of the outer wall of the 

 infraorbital canal is also characteristic ; it is a stout, more or less 

 vertical plate of bone (the " masseteric plate " or " descending 

 ramus of the maxillary root of the zygoma "), which is always 

 placed more or less transversely or obliquely to the long axis of the 

 skull; the front edge is slightly emarginate or undercut and 

 never projects in advance of the front border of the superior 

 ramus of the maxillary root of the zygoma. 



The infraorbital canal itself is formed essentially as in other 

 Muridae, its upper portion being widened for the transmission of a 

 slip from the masseter medialis muscle, its lower portion, narrow 

 and slit-like, serving for the passage of the facial or infraorbital 

 branch of the superior maxillary or second division of the fifth 

 nerve and the accompanying vessels; in Ellobius, however, 

 the lower part of the canal is reduced and closed, but the upper 

 25ortion, in compensation, is somewhat more spacious than usual. 



The zygomatic arches are strongly built and more or less 

 widely bowed laterally; each is greatly strengthened by the 

 oblique position and peculiar shape of its lower maxillary root 



