_Pipistrellus cinnamomeus_ Miller, Proc. Acad. Nat. Sci. Philadelphia, p. 390, September 3, 1902, type from Montecristo, Tabasco (preoccupied by _Vespertilio cinnamomeus_ Wagner, Schreber's Saeugethiere, suppl., 5:755, 1855, a renaming of _Vespertilio ruber_ E. Geoffroy Saint-Hilaire). _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. [Illustration: 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. [Illustration: FIGS. 2-5. Four views of the skull of _Myotis fortidens_. No. 32112, University of Kansas Museum of Natural History, [M], obtained 20 kilometers east-northeast Jesus Carranza, 200 feet elevation, Veracruz, Mexico, on May 16, 1949, by Walter W. Dalquest; original no. 12869. x2.]