_Remarks._--_Myotis fortidens_ is known only from the Tropical Life-zone. The skin, without a skull, from Esquinapa, Sinaloa, agrees in color with the undoubted specimens of _M. fortidens_ from Papayo, Guerrero, but can be matched also by selected skins of _Myotis occultus_ from Blythe, Riverside County, California. Without the skull the reference of this specimen to _M. fortidens_ is provisional. Reason for referring it to _fortidens_ rather than to _M. occultus_ is provided, however, by a series of eleven specimens of _M. occultus_ from Alamos, Sonora. These are Saccardo's Umber rather than Cinnamon-Brown and they are geographically intermediate between the reddish _M. occultus_ of California and the reddish _M. fortidens_ of Mexico. Furthermore, these specimens from Alamos have large skulls of slightly different proportions than those of _M. fortidens_ or than those of _M. occultus_ from California; possibly the animals from Alamos are representative of the larger, duller-colored variation for which Hollister proposed the name _Myotis baileyi_ (Proc. Biol. Soc. Washington, 22:44, March 10, 1909). This duller-colored type of animal intervenes between the geographic ranges of undoubted _M. occultus_ and undoubted _M. fortidens_. The specimen from Esquinapa, in the geographic sense, is on the _fortidens_ side rather than on the _occultus_ side of the _baileyi_ population. This geographic position is the basis on which the specimen from Esquinapa is referred to _M. fortidens_. The third premolar is lacking from each side of both the upper and the lower jaws of each individual of this series from Alamos. The specimens of _M. fortidens_ are all distinguishable by their color from other kinds of _Myotis_ found in the same area. Occasional individuals of _Myotis velifer_, as for example three from Las Vigas, Veracruz, also are reddish but they are of brighter tone. In addition, the larger size and cranial features of these specimens of _M. velifer_ permit ready differentiation of them from specimens of _M. fortidens_. One specimen (No. 32113) of _M. fortidens_ from twenty kilometers east-northeast of Jesus Carranza is lighter than the others, being near (_j_) Cinnamon-Brown above and is lighter on the under-parts than on the upper parts. Another individual (No. 32112) is duller colored than the others, being Snuff Brown both above and below. Otherwise the specimens of _M. fortidens_ agree in color.