6 BULLETIN 144, UNITED STATES NATIONAL MUSEUM 



pattern is often a convenient character by which to distinguish a 

 Myotis from a Pipistrellus in parts of America where the two genera 

 occur together. One North American species of Myotis, however 

 {M. sodalis), has definitely tricolored dorsal fur, but the pattern is 

 not so noticeable as it is in the species of Pipistrellus with which the 

 animal is associated. In the pallid desert race of Myotis calif omicus 

 an obvious tricolor pattern is also met with. 



The actual color is never bright. It ranges in different species 

 from a very dark, almost blackish brown {niffricans, cMloen-sis) 

 through dull brown to a decided reddish brown {t^her, calif omicus), 

 or, in desert races, to a very pale buff. The underparts are usually 

 paler than the back, sometimes rather noticeably so (albescens), 

 but there is never any striking color pattern. The ears and mem- 

 branes vary from dull brownish to whitish or to almost black, some- 

 times (albescens, nigricans) nearly concolor with the back, some- 

 times (evotis, subulatus Say, not of H. Allen) strongly contrasted 

 with it. Immature individuals are normally darker and duller than 

 adults. No sexual differences in color have been detected, but two 

 color phases independent of sex — a darker and a lighter, or duller 

 and brighter — may not infrequently be observed. For example, in 

 M. lucifugus an olive and a bronzy phase are present. In the 

 South American M. ruber there is likewise the dark brown and the 

 rusty phase ; and rusty individuals of M. nigricans occasionally occur. 

 In M. evotis as well as in M. suhulatus (Say) the black pigmenta- 

 tion of the ears and membranes is a character usually retained even 

 under desert conditions that bleach the color of the fur, while in the 

 arid-country forms of M. lucifugus the ears are less intensely pig- 

 mented and the interfemoral membrane becomes whitish, with often 

 a contrasting white border along the fr'ee edge of the wing. 



Color differentiation among the species of Myotis is not very 

 great. In the six species known to occur in the eastern United States 

 the general brownish color is so nearly alike that it is often difficult 

 to identify individual specimens by this character alone, although 

 a comparison of enough skins shows that each animal has its dis- 

 tinctive hue. This general similarity has led to great confusion in 

 the identification of specimens. 



Skull. — In both form and size the skull presents variations which 

 are useful in distinguishing species. Plate 1 has been prepared with 

 the special object of giving a clear idea of these features. The great 

 difference in size which separates Pizonyx from the largest known 

 American Myotis is at once evident on comparison of Figure 1 

 (Pizonyx) with Figure 4 (Myotis velifer incautus). Figures 4 and 

 5 show the extremes of known variation in size of skull among the 

 American species of Myotis (M. velifer incautus and M. nigricans 

 dominicensis) . Further inspection of the plate shows that the 



