Type.—Female, adult, skin and skull, No. 55497 Museum of Natural History,
The University of Kansas; from one mile east of La Pesca, Tamaulipas, México;
obtained on May 27, 1953, by Gerd Heinrich, original number 6933.

Diagnosis.—Size medium for species (see measurements); general color
cinnamon buff, almost pure on dorsal surface of hind foot and dorsal and ventral
midline of tail; spots well marked; postorbital constriction narrow; auditory
bullae small; viewed from front, lower border of maxillary plate forming continuously
concave line with jugal; zygomatic process of maxillary narrow, having
anterior border concave.

Comparisons.—From Spermophilus spilosoma annectens (specimens from
Padre and Mustang islands, Texas), S. s. oricolus differs as follows: larger in
external dimensions but smaller in cranial dimensions; postorbital constriction
narrower, 13.4 (13.2-13.8) instead of 14.0 (13.5-14.7); rostrum slightly
broader; greatest distance between posterior border of maxillary plate and
squamosal arm of zygoma longer, averaging 9.9 (9.7-10.2) instead of 9.6
(9.0-10.1); zygomatic process of maxillary, viewed dorsolaterally above lacrimal
narrower, having anterior border concave instead of almost straight; when skull
viewed from front, lower border of maxillary plate forming continuously concave
line with jugal instead of almost a right angle where jugal and maxillary
meet; ramus of lower jaw narrower; general color paler; upper surface of hind
foot washed with cinnamon instead of yellowish; postauricular spot absent
(usually present in annectens).

From Spermophilus spilosoma pallescens (specimens from southeastern
Coahuila), S. s. oricolus differs as follows: color more cinnamon especially on
upper surface of hind foot; dorsal spots more distinct; smaller externally, except
hind foot, which measures the same; braincase and postorbital constriction
narrower; nasals shorter; auditory bullae conspicuously smaller.

From Spermophilus spilosoma cabrerai (specimens from eastern San Luis
Potosí), S. s. oricolus differs mainly in pale (cinnamon) rather than dark
(blackish) upper parts, but differs also in being smaller externally (judging
from measurements given in the original description by Dalquest, Proc. Biol. Soc.
Washington, 64:107, 1951), and in having a greater zygomatic breadth, narrower
braincase and postorbital constriction, and narrower maxillary process.

Measurements.—Average and extreme measurements of eight specimens from
the type locality (four females, including type, and four males) are as follows:
total length (only five specimens), 234 (212-245); length of tail vertebrae
(five only), 67 (50-75); length of hind foot, 36.1 (35-37); length of ear from
notch, 8.4 (7.5-10.0); length of head and body, 166 (155-171); greatest length
of skull, 41.2 (40.6-42.7); zygomatic breadth, 24.4 (23.7-25.2); cranial breadth,
18.4 (17.8-18.9); interorbital constriction, 9.2 (8.5-9.8); postorbital constriction,
13.4 (13.2-13.8); length of nasals, 14.0 (13.6-14.8); length of maxillary
tooth-row, 8.0 (7.7-8.4); greatest distance between posterior border of maxillary
plate and squamosal arm of zygoma, 9.9 (9.7-10.2).

Remarks.—The type locality of S. s. oricolus is nearly at sea level on the
coastal plain of eastern Tamaulipas. In so far as now known, the population
of ground squirrel here named as new is isolated, the nearest records of occurrence
being those reported by Hall (loc. cit.) from a place on the barrier
beach approximately 80 miles north of La Pesca. The nearest record from
the mainland (S. s. annectens) is from southern Texas.

A possible explanation for the presence of the species at La Pesca is that
it dispersed southward along the barrier beach, and that an isolated or semi-isolated
segment finally reached the mainland where the barrier beach rejoins
it just northeast of La Pesca. This possibility is strengthened by study of
the specimens already mentioned from 88-89 miles south and 10 miles west
of Matamoros, because they combine many characters of annectens and
oricolus. The insular specimens differ from both annectens and oricolus in
having shorter nasals and a shorter skull, narrower zygomatic and interorbital
regions, and a relatively broader interpterygoid space. Also, the zygomatic
process of the maxillary, viewed dorsolaterally above the lacrimal, is even
narrower that in oricolus and has the anterior border even more concave. In
color, specimens from the barrier beach are pale as is oricolus, but the over-all
color is reddish cinnamon rather than cinnamon buff. Concerning the juncture
of the zygomatic plate with the jugal, 12 of the 13 specimens studied
resemble annectens in this character and one resembles oricolus. The specimens
from the barrier beach may themselves represent an unnamed subspecies;
more material than now is available is needed, because most of the
specimens are not fully adult. Because the 13 specimens from the barrier
beach resemble oricolus slightly more than annectens, all characters considered,
they are tentatively assigned to oricolus.

The author is grateful to Professor E. Raymond Hall and Mr. J. Knox
Jones, Jr., for permission to examine the specimens here reported and for
helpful suggestions. Field work that yielded the specimens was financed by
the Kansas University Endowment Association. The laboratory phases of the
study were made when the author was a half-time Research Assistant supported
by a grant, No. 56 G 103, from the National Science Foundation.

Specimens examined.—A total of 23, all from Tamaulipas: 88 mi. S, 10 mi.
W Matamoros, 12; 89 mi. S, 10 mi. W Matamoros, 1; 1 mi. E La Pesca, 10.

Transmitted November 8, 1961.