A REVIEW OF SOME BIVALVE SHELLS OF THE GROUP ANATINACEA FROM THE WEST COAST OF AMERICA. By William Healey Dall, Curator of Mollushs, United States National Museum. This group of bivalves contains species which are in general very similar, and often possess analogues in different f aunal regions which have been confounded under a single specific name. Like most genera, when the specific characters are rigorously analyzed, the species separate into distinct groups arranged geographically in ac-cordance with the general laws governing the distribution of moUusks. Not a single Atlantic species of the temperate regions has so far been found on the Pacific coast of either American continent. Even the species which inhabit the western Arctic Ocean are mostly dis-tinct from those of Greenland and Europe. Family THRACIIDAE. Taking up the Thracia family,-we find that Cyathodonta is a very ancient group and species are found in the Antillean Oligocene. Typical TJiracia seems to have come in in Eocene time. If, as claimed by Zittel, the Triassic Corimya of Agassiz is a Thracia, the group had its inception in the Mesozoic. The earliest name for the latter is Rupicola, Fleuriau de BeUevue, 1802. This name, however, is preoccupied for a genus of birds, dating from 1760. Recluz ^ has described as existing between Thracia and Rupicola anatomical differences in the giUs, foot, and siphon, which, if confirmed, would separate the two groups generically if not more widely. But I am unable from an examination of the literature of the subject to find satisfactory confirmation of these differences, which may have been due in the case of the Recluz specimen to mu-tilation or abnormahty. Ixartia Leach, Rwpicilla Schaufuss, and possibly Pelopia H. and A. Adams are s5nionymous with Rupicola Fleuriau. For the true Thracia Blainville, in the errata to his Manual (p. 600), restricts his group to that typified by T. corhuloidea, remov-ing his division B, typified by T. pubescens to the genus Osteodesma Deshayes. > Journ. de Conchyl., vol. 4, p. 120, 1853. Proceedings U. S. National Museum, Vol. 49-No. 2116. 441
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