This paper is one of a series dealing especially with the stratigraphy and paleontology of the marine Oligocene formations of western Oregon. 1 In order to appreciate the biostratigraphic significance of the common occurrence of specimens of Acila in these formations, a review of all the species of this genus, named by H. and A. Adams in 1858, was undertaken, and some of the results are presented here. Two subgenera of this genus of nuculid pelecypods are recognized: Acila, sensu stricto (typified by Nucula divaricata Hinds), and Truncacila Schenck, 1931 (typified by Nucula castrensis Hinds). Truncacila ranges in time from late early Cretaceous to the present day; Acila, s. s., from Oligocene to Recent. The modem species are restricted to the Pacific and Indian oceans. Truncacila lives off Zanzibar, in the Bay of Bengal, off certain of the Philippine Islands, in the waters of the Japanese Empire, and off the west coast of North America from Alaska to San Diego. Acila, s. s., is today restricted to the western Pacific Ocean. Notes on the anatomy, habitat, and locomotion are included in the description of the animal. Discussion of shell morphology deals with the terms employed in descriptions. The definitions of the genus and subgenera are followed by notes on the criteria used in identifying species. Hypothetical migrations are based (1) upon the geographic distribution of Recent species, and (2) upon the geologic ranges of the various species. One migration was along a Tethian route during Cretaceous time; another was a trans-polar . . .