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. 



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