Of the genera of rock-boring lamellibranchs which occur in British waters, Hiatella (= Saxicava) is the commonest. But the method of boring remains obscure, and for this reason the present research was suggested by Professor C. M. Yonge, under whose direction it was conducted. Observations were made during 1945 and 1946 in the Clyde Sea Area and at the Millport Laboratory, while other work on living and preserved material from both the Clyde and Plymouth areas was carried out at the Department of Zoology, University of Glasgow. Acknowledgments are due for assistance in technical problems to Dr H. F. Steedman of the University of Glasgow, for help with the nomenclature of the genus to Mr R. Winckworth, and for much kindness and help to the late Mr R. Elmhirst, Director of the Millport Laboratory.