Study of these small bivalves is handicapped by a number of factors. The two valves are usually very differently sculptured and although individual left and right valves are often obtained in dredgings, associated valves are very rarely collected. There is therefore inherent difficulty in associating left and right valves correctly. In transparent valves the sculpture is often very fine, and different elements are stressed according to whether the specimens are examined by transmitted or reflected light. In addition, if reflected light is used at different angles different sculptural elements come into prominence. The right valve is usually transparent when young, but, as the shell grows, a white, opaque deposit is formed on the interior of the valve and gradually covers the whole shell. This has the effect of cloaking those elements that can be studied by transmitted light and stressing the external raised sculpture. In the left valve both the strength and spacing of concentric sculpture may vary considerably in one population so that these characters alone are not adequate for species differentiation. More species occur in New Zealand waters than have been recognized previously so that extreme care is necessary in studying this group. Because of the difficulties outlined above it is apparent that the systematica of the group should in most cases be based upon the characters of one valve and the left valve is obviously the one that should be selected.