The molluscan faunas north of the Rio Grande and those of theTampico Embayment have been fairly well known for many years. The intermediate faunas between the Rio Grande and the Rio Conchos were untouched until the interest in petroleum investigations was focused on northeastern Mexico. The report is a systematic study of molluscan collections made in the course of the geological field work. Old species are discussed, and new species from the Eocene, Oligocene, and Miocene series are described and figured.
An attempt is made to relate them to the faunas of known areas. A few introductory paragraphs have been written on the historical development of the area and on the stratigraphic background.