two dunes. The
windward side of a dune was toward the Gulf and the slope of that side
was gentler than that on the leeward side. According to the cycle
described by Davis (Proc. Amer. Acad. Arts and Sciences, 22:303-332,
1896) and recently figured on page 364 by Lobeck (Geomorphology, 1st
ed., xii + 731 pp., 1939, McGraw Hill Book Co., Inc., New York) the
barrier beach concerned was in the early part of the "Middle Youth
Stage".

Typically, on the center of the area in the lee of a dune there was a
patch of plum brush, almost five feet tall and so dense that a person
could not penetrate it. A belt of grass, 20 to 100 feet wide, surrounded
the plum brush. The grass was approximately 20 inches high. Outside the
area of grass, there were widely-spaced xerophitic shrubs which grew
also on the dunes. The diagram (fig. 1) shows these prominent features
as a person might see them if he looked directly down from an airplane.

We obtained specimens of the spotted ground squirrel (_Citellus
spilosoma_), Ord kangaroo rat (_Dipodomys ordii_), hispid cotton rat
(_Sigmodon hispidus_) and black-tailed jack rabbit (_Lepus
californicus_). Tracks and other sign of the coyote (_Canis latrans_)
were seen. So far as we could ascertain, by our own investigations and
from our Mexican hosts at the fishing camp, no other kinds of native
mammals lived on the island. The ground squirrel and kangaroo rat were
found by us on only the sandy areas where there were xerophitic shrubs.
The cotton rat was found only in the grass. The jack rabbit and coyote
ranged over the whole of the island excepting the areas of plum brush in
which we saw no sign of any mammal.

To answer the second of our initial questions: The affinities of the
mammals of the barrier beach of Tamaulipas are approximately equally
divided between those of the mainland and those of Padre Island. The
ground squirrel is indistinguishable from the subspecies which occurs
both on the mainland and Padre Island to the northward; the other three
kinds of mammals of which we obtained specimens prove to be
subspecifically distinct from any previously named kinds and seem to be
confined to the off-shore beach. Accounts of these four mammals and of a
previously unnamed subspecies of kangaroo rat on Mustang Island, Texas,
follow.

Citellus spilosoma annectens (Merriam)

Spotted Ground Squirrel

1893. _Spermophilus spilosoma annectens_ Merriam, Proc. Biol. Soc.
Washington, 8:132,