_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._ 29-1505 * * * * * Transcriber Note: Italic text is denoted by _underscores_ Bold text is denoted by =equal signs=