4 BULLETIN 144, UNITED STATES NATIONAL MUSEUM 



DIFFERENTIAL CHARACTERS OF THE AMERICAN SPECIES OF MYOTIS 



As the American members of the genus Myotis show no excessive 

 developments of structure, the differential characters by which the 

 species may be recognized are not always readily understood. The 

 more important of these characters will therefore be passed briefly 

 in review. 



Ear. — The ear is usually about long enough to reach the end of 

 the muzzle when laid forward. In Myotis volans it is slightly 

 shortened, but in such species as M. keenii { = M. subulatus H. Allen 

 and recent authors, not of Say), M. evotis^ M. thysanodes^ and M. 

 chiloensis, it is elongated suflEiciently to extend rather noticeably be- 

 yond the tip of the muzzle. The tragus is of characteristic slender 

 lancet shape, varying little in its proportionate height, which is 

 nearly half that of the ear. While the length of the ear relatively 

 to that of the head is often a very important specific character, we 

 have found the actual form of less value in distinguishing between 

 nearly related members of the genus. 



Wing and membranes. — Usually the third to fifth metacarpals and 

 third to fifth fingers are slightly graduated; but in a few species 

 these metacarpals tend to be nearly equal, as are also the fourth and 

 fifth digits. Otherwise we have not found that characters derived 

 from the bony elements of the wing are of much service in determin- 

 ing species. This remark, of course, is not intended to allude to the 

 actual lengths of forearms and of digits, as these measurements are 

 very significant. 



The point of attachment of the wing membrane to the lower limb, 

 a feature of importance in the Old World members of the genus, is 

 a character which does not contribute much to the determination of 

 American species of Myotis. In all of these except the North Amer- 

 ican M. grisescens and the South American M. mnus (also M. 

 pilostvs, if this is actually an American bat; see pp. 25, 208) the 

 membrane is inserted on the side of the foot near the base of the 

 phalanges ; in M. grisescens and M. simus the point of insertion is at 

 the ankle. While the type of insertion thus remains surprisingly 

 constant in the American species of. Myotis., a very peculiar condition 

 has been developed in Pizonyx (see p. 33). 



Foot. — The length of the foot relatively to that of the tibia varies 

 much both individually and from species to species. When a few 

 specimens of one kind are measured the variations in ratio of foot 

 length to tibia length will be found to group themselves about a mean 

 which is an important specific character. In the American species 

 the means of this ratio range from about 40 to about 60. 



