Inspection of fauna lists suggests that many authors have found difficulty in identifying the British species of Thracia, most being content to list them as Thracia spp. Examination of museum collections and even keys to identification (Ziegelmeier, 1957) shows many errors of identification. The problem was brought to notice when some specimens dredged off the Northumberland coast appeared to differ from the five species recognized as occurring in British waters. It was decided that a study of the specific differences similar to that of Holme (1951, 1959) for Ensis and Lutraria, and Allen (1954) for Nucula, would be useful.

Specimens, particularly undamaged ones, of the British species of Thracia are difficult to obtain because they are both fragile and rare, and I am greatly indebted to the many people who were so kind as to give material and information for this study. These included Mr J. H. Barrett, Dale Fort Field Centre; Dr C. Burdon-Jones, Marine Biology Station, Menai Bridge; Mr I. C. J. Galbraith, British Museum (Nat. Hist.); Prof. R. A. R. Gresson, Queen's University, Belfast; Prof. L. A. Harvey, The University, Exeter; Mr N. A. Holme, Marine Biological Association, Plymouth; Dr N. S. Jones, Marine Biological Station, Port Erin; Mr A. D. Mclntyre, Scottish Home Department, Aberdeen; Mr R. McLaughlin, The University, Aberdeen; Dr R. H. Millar and Dr R. B. Pike, Scottish Marine Biological Association, Millport; Mr A. M. Tynan, Hancock Museum, Newcastle upon Tyne. I also wish to thank Dr D. A. Edwards, King's College, Newcastle upon Tyne, for assistance in matters mathematical.