1949, by Walter W. Dalquest; original no. 12869. ×2.

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 Álamos, 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 Álamos have large skulls of slightly different proportions than those of M. fortidens or than those of M. occultus from California; possibly the animals from Álamos 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 Álamos.

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 Jesús 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.

Among named kinds of Myotis, M. fortidens resembles Myotis lucifugus and Myotis occultus. From the former, M. fortidens differs in possessing a strong sagittal crest and in lacking the third premolar in both the upper jaw and the lower jaw. M. fortidens lacks the glossy sheen found on the pelage of many individuals of M. lucifugus. From M. occultus, M. fortidens differs in having the rostrum (viewed from above) smaller in relation to the braincase. This is true of specimens with the teeth showing much wear as well as in specimens with the teeth unworn or only moderately worn. Also, M. fortidens is longer bodied as may be seen by comparing the measurements given here with those recorded for M. occultus by Miller and Allen (Bull. U. S. Nat. Mus., 144:100, May 25, 1928). We are agreed that M. fortidens is as closely related to M. occultus as to any other named kind of Myotis, and that it is more closely related to it than to most other species of the genus, but one of us (Dalquest) thinks that M. fortidens is specifically distinct from M. occultus, whereas the other author (Hall) inclines to the view that additional specimens from localities intermediate between the known geographic ranges of M. occultus and M. fortidens will