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 Jesús 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
| No. | Sex Age | Locality | Greatest length | Condylobasal length | Zygomatic breadth | Interorbital constriction | Breadth of braincase | Mandible | Maxillary tooth-row | Maxillary breadth at M3 | Mandibular tooth-row | Wear of teeth |
| 25030 | ♂ | Esquinapa | .... | .... | ... | ... | ... | .... | ... | ... | ... | ? |
| 126650 | ♀ | Papayo | 15.0 | 14.2 | 9.7 | 3.9 | 7.1 | 11.5 | 5.5 | 5.6 | 6.0 | 0 |
| 126651 | ♀ | Do. | 15.1 | 13.8 | 9.4 | 3.8 | 6.8 | 10.6 | 5.6 | 5.9 | 6.0 | 0 |
| 17834 | ♂ | P. Negras[1] | .... | 4.1 | 10.6 | 5.6 | 5.7 | 6.0 | 0 | |||
| 17835 | ♀ | Do. | 15.5 | 14.9 | 9.6 | 4.2 | 7.2 | 11.0 | 5.7 | 6.0 | 6.1 | 2 |
| 17836 | ♀ | Do. | 15.5 | 14.5 | 9.7 | 4.2 | 7.3 | 10.9 | 5.4 | 5.9 | 5.7 | 3 |
| 32112 | ♂ | J. Carranza[2] | 15.3 | 14.4 | 9.7 | 4.1 | 7.3 | 11.5 | 5.7 | 5.9 | 6.3 | 1 |
| 32113 | ♂ | Do. | 15.0 | 14.0 | 9.5 | 4.2 | 7.2 | 10.9 | 5.5 | 5.9 | 5.9 | 1 |
| 32114 | ♂ | Do. | 15.0 | 13.9 | 9.7 | 4.1 | 7.2 | 10.8 | 5.4 | 6.0 | 5.9 | 1 |
| 88.8.8.18 | ♀[3] | Teapa | 15.0 | 13.8 | 9.6 | 3.8 | 7.4 | .... | 5.4 | 5.8 | 5.8 | 1 |
| 100231 | ♀[4] | Montecristo | 15.0 | 14.1 | 9.0 | 4.0 | 7.2 | 11.4 | 5.8 | ... | 6.0 | 0 |
| Average | 15.2 | 14.2 | 9.5 | 4.0 | 7.2 | 11.0 | 5.6 | 5.9 | 6.0 |
[Note 1: 20 km. WNW Piedras Negras.]
[Note 2: 20 km. ENE Jesús Carranza, 200 ft.]
[Note 3: Type of Myotis lucifugus fortidens; measurements after Miller and Allen, Bull. U. S. Nat. Mus., 144:100; 101, May 25, 1928.]
[Note 4: Type of Pipistrellus cinnamomeus Miller 1902.]