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 reveal intergradation between the two kinds. However that may be, there is no proof at present of such intergradation and the binomial is therefore used for the Cinnamon Myotis. _Specimens examined._--Total number, 10, all from Mexico, each a skin with skull except the skin-only from Sinaloa. _Sinaloa_: Esquinapa, 1 (Amer. Mus. Nat. Hist.). _Guerrero_: Papayo, 2 (U. S. Biological Surveys Collection). _Veracruz_: 20 km. WNW Piedras Negras, 3 (Mus. Nat. Hist., Univ. Kansas); 20 km. ENE Jesus Carranza, 200 ft. elevation, 3 (Mus. Nat. Hist., Univ. Kansas). _Tabasco_: Montecristo, 1 (U. S. Biological Surveys Collection). _Additional record._--Tabasco: Teapa, the holotype of _Myotis lucifugus fortidens_ Miller and Allen 1928. _University of Kansas Museum of Natural History, Lawrence, Kansas. Transmitted October 31, 1949._ CRANIAL MEASUREMENTS OF _Myotis fortidens_ Column headings: A: Greatest length B: Condylobasal length C: Zygomatic breadth D: Interorbital constriction E: Breadth of braincase F: Mandible G: Maxillary tooth-row H: Maxillary breadth at M3 I: Mandibular tooth-row J: Wear of teeth