On page 392 of the original description--which our study of the holotype shows to be accurate--Miller wrote: "This bat differs so widely from the other known American species of _Pipistrellus_ as to need no special comparisons. Superficially it has much the appearance of an unusually red _Myotis lucifugus_, and only on examination of the teeth do the animal's true relationships become apparent." In referring to the teeth Miller almost certainly was thinking of the premolars of which there are only two on each side of the upper jaw and on each side of the lower jaw in _Pipistrellus_, including his _Pipistrellus cinnamomeus_, whereas _Myotis_ at that time was thought always to have three premolars on each side of both the upper and lower jaw, except in rare instances where one premolar might be lacking on one side of one jaw or even more rarely on both sides of the upper jaw. In his original description of _P. cinnamomeus_, Miller mentioned also that it had the "Inner upper incisor distinctly smaller than the outer, not approximately equal to it as is the case in _P. subflavus_." At this point it is well to make clear that each of the genera _Pipistrellus_ and _Myotis_ contains a large number of species and that the differences between the two genera are few. Our examination of American specimens reveals only one differential character: In _Myotis_ the outer upper incisor is distinctly larger than the inner, whereas the two incisors are of approximately equal size in _Pipistrellus_. It may be noted that the outer upper incisor of several, but not all, species of _Myotis_ has a well-developed concave surface directed toward the canine whereas this surface is flat or convex in _Pipistrellus_. In both features, the type of _Pipistrellus cinnamomeus_ Miller agrees with _Myotis_ and differs from _Pipistrellus_. Five years after naming and describing _Pipistrellus cinnamomeus_, Miller published his monumental work entitled "The families and genera of bats" (Bull. U. S. Nat. Mus., 57, June 29, 1907) wherein he points out the differences in the upper incisors between _Pipistrellus_ and _Myotis_ (by a _lapsus plumae_ ascribes subequal incisors to _Myotis_ and unequal incisors to _Pipistrellus_) but seemingly failed to reexamine _P. cinnamomeus_ in the light of this better understanding of the two genera, or if he did examine _P. cinnamomeus_ he possibly was misled still by the absence of the third premolar on each side of both the upper and lower jaw.