ON THE NATURE AND DEFINITION OF THE LUNULE, ESCUTCHEON AND CORCELET IN THE BIVALVIA

'Before I begin to detail my system, I think it very necessary to settle the technical terms or names for the parts of the bivalves, useful for making their descriptions intelligible and easy, as also decent; for I hold in great detestation the obscene terms made use of by the Linnean School ' (Da Costa, 1776, p. 239). Textbooks of palaeontology have for many years included the terms lunule, escutcheon and corcelet in their treatment of general morphological terms used in Bivalvia. In addition, many systematic descriptions of new taxa in this class incor-porate these terms as an essential part of the descriptive terminology of the bivalve shell. It is strange that no modern work has been done on either the detailed morphology of these structures or on their possible significance and function. TERMINOLOGY OF SHELL STRUCTURE Until the recent work of Oberling (1955, 1964) most workers on the shell structure of the Bivalvia used a generalized form of nomenclature, describing the macro-scopically distinguishable layers of the shell as inner, middle and outer shell layers respectively. Oberling (1955) proposed the terms ectostracum (for outer layer), mesostracum (for middle layer), and endostracum (for inner layer). These shell layers may be clearly distinct, or sometimes combined (e.g. mes-ectostracum). He also recognized as distinct those calcareous layers which are secreted at the site of muscle attach-ments—terming these myostraca of various types. The term periostracum was retained for the thin conchiolin layer covering the outer surface of the shell. I have found these terms to be useful in that they are far more concise than the older terminology. However, they do tend to suggest an element of precision not implicit in the older terminology; this is deceptive precision as the new terms are exact equivalents of the previously used ones, as Oberling himself makes clear (1964, pp. 5-6). I do not suggest, by using these terms descriptively for the different major groups of bivalves, that they are necessarily exactly homologous throughout the class. at Jam es Cook U niversity on D ece