Myotis lucifugus fortidens Miller and Allen, Bull. U. S. Nat. Mus., 144:54, May 25, 1928.
Type.—"Adult female (in alcohol) No. 88.8.8.18, British Museum (Natural History). Collected at Teapa, Tabasco, Mexico, by H. H. Smith, January 5, 1888. Presented by Messrs. Salvin and Godman [after Miller and Allen, Bull. U. S. Nat. Mus., 144:54, May 25, 1928]."
Range.—Known only from the lower part of the Tropical Life-zone of the region of the Isthmus of Tehuantepec and east and west coasts of Mexico.
Fig. 1. Map showing localities from which Myotis fortidens has been recorded.
Diagnosis.—Among American species of the genus, over-all size medium (total length 94 mm); body long (54); tail short (39); forearm of medium length (37); tibia short (14.5); foot long (58 per cent of length of tibia); wing membrane arising from side of foot at distal end of metatarsal; calcar simple (not keeled) and 7 mm long; ears 15 to 16 mm long measured in the flesh from the notch (posteroventral border of the meatus); tragus, measured from same place, 7 to 8 mm high with posterobasal lobe; third metacarpal longest and second metacarpal shortest; fifth shorter than fourth; ears brownish; membranes of wing and tail blackish; uropatagium almost hairless, the few hairs that are present being almost invisible; pelage of back 5 mm long with some overhairs 8 to 9 mm long; basal 3 mm of fur black, remainder Cinnamon-Brown (capitalized color terms, after Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912); outline of skull viewed dorsally similar to that of Myotis lucifugus; sagittal crest well developed; distance across upper canines equal to or slightly exceeding interorbital constriction; braincase low; two premolars on each side in upper jaw and also in lower jaw, the one remaining small premolar in contact with both the canine and the fourth premolar.
Figs. 2-5. Four views of the skull of Myotis fortidens. No. 32112, University of Kansas Museum of Natural History, ♂, obtained 20 kilometers east-northeast Jesús Carranza, 200 feet elevation, Veracruz, Mexico, on May 16, 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.