This is a zoological book. What species of mammals occur on the 'coastal island', barrier beach, of Tamaulipas? Are the closest relatives of these mammals on Padre and Mustang islands of Texas, instead of on the mainland of Tamaulipas, or are the mammals on the barrier beach distinct from all others? These were questions that Dr. von Wedel of Oklahoma City and I asked ourselves in March of 1950 when we were in southern Texas. With the aim in mind of answering these questions, Dr. von Wedel arranged round-trip transportation, by air, for the two of us between Brownsville, Texas, and Boca Jesús María. The latter place is a 'pass', tidal inlet, through the long barrier beach. The waters of the Gulf of Mexico and of the lagoon behind the beach flow back and forth with the changing tides through the inlet. We arrived at Boca Jesús María on March 18, 1950, and left 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 89½ 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.