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.
In each of our two specimens there is no sagittal crest but instead a low ridge one millimeter wide which marks the space between the margins of the two temporal muscles.
Allusion already has been made to the resemblance of the newly named Myotis argentatus to the silvery-haired bat, Lasionycteris noctivagans (LeConte). The whitish tips of the hairs are slightly more yellowish in argentatus but the difference is so slight as to be detected by only the most careful comparison. The remainder of the pelage in argentatus is black as in the darkest individuals of Lasionycteris.
Among named kinds of the genus Myotis, the species argentatus most closely resembles Myotis albescens which, up to now has been recorded from as far south as Argentina, in South America, and as far north as Nicaragua, in Central America (Miller and Allen, Bull. U. S. Nat. Mus., 144:202, 203, 1928). The differences detected between the two species are indicated above in the paragraph of comparisons and some other differences can be detected by comparing measurements given below with those of M. albescens as recorded by Miller and Allen (op. cit.: 204-205). In initial comparisons with albescens, only Paraguayan specimens were employed. It was felt that specimens of albescens from the northernmost localities of occurrence might more closely resemble argentatus. Accordingly, we appealed a second time to Dr. A. R. Kellogg for comparative material and he lent us the specimens (alcoholics with skulls separate) in the U. S. National Museum from Central America. These also differ from our newly named bat in the same fashion as do the South American specimens. Further, the number and magnitude of the differences between albescens and argentatus greatly exceed any that can be pointed to between the American subspecies of any other one full species of the genus Myotis. Full specific, rather than mere subspecific, status, therefore, is suggested for the bat here named Myotis argentatus.
Measurements.—The adult, male type, and the immature female specimen measure, respectively, as follows: Head and body, 55, 51 mm.; tail, 32, 29; tibia, 13.7, 13.2; foot, 8, 9; forearm, 33.0, 34.5; thumb, 5.8, 5.7; third metacarpal, 32.2, 30.5; fifth metacarpal, 31.5, 30.3; greatest length of skull, 14.5, 14.0; condylobasal length, 13.8, 13.0; zygomatic breadth, 9.1, 9.0; interorbital constriction, 4.3, 4.0; breadth of brain case, 7.5, 7.4; occipital depth, 5.7, 5.7; mandible, 10.5, 10.0; maxillary tooth row, 5.3, 5.0; maxillary breadth at M3, 5.5, 5.7; mandibular tooth row, 5.6, 5.3.
Specimens examined.—Two, from the type locality.
Transmitted October 20, 1947.
FOOTNOTE
[A] Assistance with field work is acknowledged from the University of Kansas Endowment Association.