1893. _Arvicola (Mynomes) aztecus_ Allen, Bull. Amer. Mus. Nat.
Hist., 5:73, April 28, type from Aztec, 5900 ft., San Juan County,
New Mexico.
Allen (_loc. cit._) described this species on the basis of two
specimens from Aztec, New Mexico, and three from La Plata, New Mexico.
He characterized it as "Size large; pelage very full and soft; tail
short; skull very narrow.
"Above grayish brown with a tinge of pale buff; fur blackish plumbeous
beneath the surface, tipped with pale yellowish brown, and varied with
longer, projecting, black-tipped hairs; below grayish white, the fur
plumbeous beneath the surface and tipped with white, giving a whitish
gray effect. Feet dusky; tail dusky brown above, dull white below."
Allen identified as this species "a large _Arvicola_ from Estes Park,
Colorado, which I have before been unable to allocate. I am unable to
find that it differs in any particular from the specimens from New
Mexico." He pointed out also (_op. cit._:73-74) that "The type and only
positively identified specimen of Baird's _Arvicola modesta_ [=
_Microtus pennsylvanicus modestus_ (Baird)] from Sawatche Pass,
Colorado, is a very young specimen in poor condition. An examination of
a series of adult and young examples from the type locality will be
necessary in order to determine its relationships to _A. alticolus_ [=
_Microtus longicaudus alticolus_ (Merriam)] and _A. aztecus_."
Bailey, in his revision of the American voles of the genus _Microtus_
(N. Amer. Fauna, 17:20), showed _Arvicola modesta_ Baird to be a
subspecies of _Microtus pennsylvanicus_ but retained _Microtus aztecus_
(Allen) as a distinct species. In describing _M. aztecus_ he wrote "the
size similar to _M. pennsylvanicus_, but with shorter tail and larger
hindfoot; skull long; braincase narrow; interparietal long ..." and
remarked that "_Microtus aztecus_ belongs to the _pennsylvanicus_
group. Externally it is not very different from _modestus_, but none of
the specimens show any signs of intergradation; and the skull
characters are so well marked that there seems no doubt of its full
specific rank."
Subsequent to the publication of Allen's (_op. cit._) account and
Bailey's account (_op. cit._), additional material was collected that
helps to clarify the relationships of _Microtus aztecus_. A comparison
of six adult topotypes of _Microtus aztecus_ with a series of nine
adults of _M. p. modestus_ from 1 mi. S, 2 mi. E Eagle Nest, 8100 ft.,
Colfax County, New Mexico, with three adults from 1-1/2 mi. E Manassa,
Conejos County, Colorado, and with four adults from Saguache County,
Colorado (all in KU), reveals that the supposed "well marked" external
and cranial differences between the two forms are not nearly so evident
as was indicated by Bailey.
The cranial differences that exist between these two forms (narrower
nasals, slightly longer interparietal, slightly longer and narrower
skull in _aztecus_) are evident only as averages. Although
geographically intermediate specimens are lacking, the morphological
differences between the two kinds of animals are of the degree and kind
that separate subspecies, rather than species. We therefore judge _M.
aztecus_ (Allen) to be only subspecifically distinct from _M.
pennsylvanicus modestus_ and employ the name _Microtus pennsylvanicus
aztecus_.