40 BULLETIN 144, UNITED STATES NATIONAL MUSEUM 



may completely conceal the dark bases and give a uniformly shining 

 appearance to the coloration of the upper side. In a well-made 

 skin, this glossy sheen to the fur is usually sufficient in itself to 

 separate the species from others with which it occurs. A dark 

 shoulder spot is usually present. 



Shall. — The skull, viewed from above (pi. 1, p. 7, fig. 12), has the 

 rostrum relatively short (distance from anterior margin of alveolus 

 of inner incisor to narrowest part of interorbital constriction decidedly 

 less than that from narrowest part of constriction to lambda) and 

 narrow (maxillary breadth at m^ usually more than 1 mm. less than 

 breadth of brain case) ; anterior breadth across roots of canines 

 obviously less than interorbital constriction, and the forehead, as 

 seen in profile, has a slight gradual upward slope. The brain case 

 is somewhat flattened and subcircular in dorsal view, its area notice- 

 ably greater than that of rostrum. Even in adults the sagittal crest 

 is seldom developed (see footnote, p. 133), though it is sometimes 

 indicated by a low median ridge, which, posteriorly at least, is 

 narrow and sharply defined. The lambdoid crests usually do not 

 quite meet at the vertex. 



Teeth. — The teeth are of normal size relatively to the rostum and 

 palate (pi. 1, p. 7, fig. 36). Length of maxillary tooth row less 

 than greatest palatal width across molars. Upper molars with full 

 maximum development of secondary cusps and ridges (fig. la and 1<?, 

 p. 8) , the protoconule always present and well developed, the hypocone 

 with base well marked off from that of the protocone and with apex 

 sometimes distinct from the inner loph; cingulum variable in its 

 development, usually obvious on lingual margin of crown, but rarely 

 if ever continuous around the antero-lingual face of the protocone; 

 exceptionally the cingulum may be almost or quite absent. The two 

 small upper premolars are usually somewhat drawn inward from 

 the tooth row, but the tips of both teeth are visible from the outer 

 side. 



Remarks. — As a species Myotis lucifugus has a broader area of 

 dispersal than any other member of its genus in the Western Hemi- 

 sphere. Over an astonishingly wide range it holds its structural 

 characters very closely, with remarkably little variation in size and 

 proportions. In color, however, it shows the usual response to dif- 

 ferences in climate, tending to become dark in areas of high humidity 

 and pallid under conditions of dryness. It is usually associated with 

 at least moderate tree growth and particularly delights to feed along 

 small water courses in the forest. It is therefore less common in our 

 arid Southwest, if not altogether absent from areas of actual desert 

 conditions; in such places it is outnumbered by the smaller, duller- 

 colored M. 'i/umanensis. In many localities, however, the two species 

 occur together along the borders of thinly wooded country, and 



