eft on March 22,
1950. Our headquarters there were in one of the four one-story buildings
immediately north of the inlet. This place is approximately 89-1/2 miles
south, and 10 miles west, of Matamoros, Mexico. Most of our collecting
was done on the sand dunes one and one-half miles north of the buildings
but on the evening of March 20 we made a round-trip, by boat of course,
to the sand dunes on the south side of the inlet to set traps; these
traps, and the _Dipodomys_ that were caught in them, were picked up the
following morning.

At the time of our visit, the part of the barrier beach south of the
tidal inlet was connected with the mainland. The connection was far to
the southward, according to our pilot, Mr. Kagy of Brownsville, and also
according to the testimony of the Mexicans at the fishing camp where we
stayed on the north side of the inlet. The barrier beach which lay to
the north of the inlet extended sixty-odd miles northward to the delta
of the Rio Grande and had, we were told, eight "passes," including Paso
Jesus Maria. At the time of our visit, however, only three of these
tidal inlets were open, it was said; the five others were thought to be
filled in with sand, which permitted terrestrial animals to move from
one part of the beach to another. Dr. von Wedel and I saw two tidal
inlets that were open when we were being flown back to Brownsville.

[Illustration: FIG. 1. Diagram of physiographic features of the barrier
beach of Tamaulipas. Top view looking down, as from an airplane, on the
beach. Bottom view is profile.]

The long, low, sandy island, technically a barrier beach, irrespective
of tide varied in width from a quarter of a mile to as much as a mile
and was separated from the mainland by the Laguna Madre, which was four
miles wide opposite our trapping station. To the northward the width of
the lagoon gradually increased until, at a place thirty miles north of
our trapping station, the lagoon was almost 20 miles wide.

The island was perhaps four feet above high tide. Superimposed on this,
in places, there were sand dunes, technically barchans, so arranged that
the end of one touched the end of the next. The tops of some were as
much as 20 feet above high tides and the chain of these connected-dunes
on which we trapped was approximately a mile long. Incipient tidal
inlets were frequent; they were where storm-driven waves of high tides
had broken across the island between the adjacent ends of