show intergradation
between _Perognathus goldmani_ and _P. artus_. From northwest to
southeast the specimens are as follows: one mile east of Buena Vista,
on Rio Yaqui Reservoir, 1000 feet (2 specimens, K. U.); Alamos, 1200
feet (7, U. S. B. S.); four and a half miles southeast of Alamos, 1000
feet (5, K. U.); nine miles southeast Alamos, 1000 feet (5, K. U.). The
specimens (_P. goldmani_) from Rio Yaqui Reservoir are largest. Those
from nine miles southeast of Alamos (_P. artus_) are smallest. Those
from Alamos proper are _P. goldmani_. Those from four and a half miles
southeast of Alamos (80051-80055 K. U. collected by Robert L. Packard
and here referred to _goldmani_) include two as large as _goldmani_
from Alamos, one as small as _artus_ from nine miles southeast of
Alamos, and two that are intermediate in size. Features other than
size, considered geographically, also suggest intergradation.

Six specimens (61409-61413, 61415 K. U. collected by J. R. Alcorn),
including five adults (permanent fourth premolar of full height and
having cusps worn but not so much as to make a lake of dentine), from
four miles north of Terrero, Sinaloa, also seem to be intergrades
between _Perognathus goldmani_ and _Perognathus artus_. As compared
with adults of _P. goldmani_ from 10 miles north-northwest of Los
Mochis, Sinaloa, and _P. artus_ from one mile south of Pericos,
Sinaloa, the specimens from four miles north of Terrero are almost
exactly intermediate in length of hind foot, width of interparietal,
and width of tympanic bullae. Intermediacy is shown also in total
length of animal (slightly nearer that of _artus_) and length of
tympanic bullae (slightly nearer that of _goldmani_). In lack of
inflation laterally of the mastoidal bullae the specimens agree with
_artus_. In occipitonasal length and mastoidal breadth the specimens
from four miles north of Terrero average even larger than _goldmani_
from 10 miles north-northwest of Los Mochis but not so large as
specimens of _goldmani_ from the type locality, which is still farther
north. The uninflated mastoidal bullae "tip the balance" slightly in
favor of _artus_ to which the specimens from four miles north of
Terrero are here referred. The degree of inflation of the mastoidal
bullae elsewhere varies geographically. For example, the mastoidal
bullae of the 25 specimens of _goldmani_ from two and a half miles
north of El Fuerte, Sinaloa, are intermediate in size as between those