_Diagnosis._--Size medium for the genus (see measurements), tail short; foot long; ears and membranes black; pelage long (maximum length on middle of back 9 mm.) and black; upper parts with overhairs tipped with whitish especially on rump; underparts from posterior part of thorax posteriorly with all of the hairs tipped with this same whitish color; skull with preorbital part small in relation to brain case; teeth small in relation to total area of palate; brain case much inflated; ventral margin of foramen magnum evenly rounded. _Comparison._--From _Myotis albescens_ (E. Geffroy) known to us by specimens in the United States National Museum from Paraguay (Tacural), Panama (Tabernilla), and Nicaragua (Prinzapolca R. and Escondido R.), _argentatus_ differs in: Body and foot longer; tail relatively shorter (57 and 58% of length of head and body versus 76 (62-83)% in _albescens_); tibia shorter; pelage longer, and black instead of brown; silver tipping of fur on hinder back markedly more conspicuous; precranial part of skull, when viewed from above, larger in relation to brain case; postorbital constriction less abrupt, that is to say, skull "longer-waisted"; occlusal surfaces of teeth of equal area and therefore occupying a relatively smaller percentage of total area of palatal surface; ventral margin of foramen magnum less deeply indented; ventrally prominent part of basioccipital twice as wide. _Remarks._--The relatively slight wear on the teeth of the female of _M. argentatus_ and the large ends on the bones of the wings indicate that it is immature. Its measurements, recorded below, average smaller than those of the adult holotype, a male, and the silvery tipping on the upper parts is almost lacking from the pelage which is shorter than in the holotype. [Illustration: FIGS. 1-6. _Myotis_. From left to right, dorsal, lateral and ventral views. All x 2. FIGS. 1-3. _Myotis argentatus_, no. 19228, Univ. Kan. Mus. Nat. Hist., type. FIGS. 4-6. _Myotis albescens_, no. 105664, [F], U. S. Nat. Mus., from Tacuaral, Paraguay; obtained on November 13, 1900, by Wm. T. Foster, orig. no. 128.] Among at least American kinds of _Myotis_, _argentatus_ is extreme in small area of occlusal surface of the upper molariform teeth in relation to the total area of the palatal surface of the skull. _M. albescens_ previously was regarded as extreme in this feature. The distance across the third upper molars, from the outside of one tooth to the outside of the other, is 5.5 mm. in the holotype of _argentatus_ and 5.4 mm. in a specimen of corresponding age and sex of _albescens_. The distance between the third upper molars, from the lingual side of one tooth to the lingual side of the other, is 2.9 mm. in _argentatus_ and 2.8 mm. in _albescens_.