nosis._--Body large for species (see tables 1 and 2); tail short;
hind foot large; upper parts Ochraceous-Buff (in summer pelage) and
Orange-Buff (in fresh winter pelage); underparts Pale
Ochraceous-Salmon; skull medium in size and narrow; zygomata narrow;
rostrum narrow; palate short; nasals medium in length; basioccipital
small and narrow; mastoid bullae not greatly inflated; tympanic bullae
inflated; incisors small; maxillary teeth small.

_Comparisons._--From _Cratogeomys castanops jucundus_, found beyond
the mountains to the southeast, _sordidulus_ differs in: Body smaller;
tail shorter; hind foot slightly smaller; upper parts darker, more
ochraceous and less yellowish, with plumbeous bases of hairs more
conspicuous; underparts darker, more buffy and less whitish; skull
slightly shorter, more nearly flat, and narrower; zygomata less widely
flaring; rostrum narrower; mastoid bullae less inflated; incisors and
maxillary teeth slightly smaller. From _Cratogeomys castanops
excelsus_, found to the south and southwest, _sordidulus_ differs in:
Body slightly smaller; tail shorter; hind foot slightly larger; upper
parts darker, more ochraceous and less pinkish-buff; underparts
darker, more buffy and less whitish; skull smaller and narrower;
zygomata less widely flaring; sides more nearly parallel and not
expanded anteriorly; palate shorter; rostrum narrower and, in relation
to greatest length of skull, longer; tympanic bullae slightly more
inflated; incisors and maxillary teeth smaller. From _Cratogeomys
castanops consitus_, found to the north and west, _sordidulus_ differs
in: Body larger; hind foot larger; upper parts paler, more ochraceous
and less rufous; skull decidedly larger and wider; zygomata more
widely flaring; palate and nasals longer; rostrum broader; mastoid
bullae and tympanic bullae larger; maxillary teeth smaller. From
topotypes of _Cratogeomys castanops clarkii_ (Baird), found to the
northwest, _sordidulus_ differs in: Body larger; tail shorter; upper
parts, in winter pelage, paler, more ochraceous and less dark-rufous;
skull slightly smaller and narrower; rostrum narrower; nasals slightly
shorter; sides of basioccipital more nearly parallel instead of
wedge-shaped; mastoid bullae less inflated; incisor and maxillary
teeth smaller. From _Cratogeomys castanops convexus_, found to the
northeast, _sordidulus_ differs in: Body larger; tail shorter; upper
parts slightly darker, more ochraceous and less buffy;