The work about to be described resulted from certain thoughts on the variation or populations or an inter-tidal bivalve mollusc of the genus Amphidesma occurring along the shores or Pegasus Bay, Canterbury. It appeared from casual observations and sampling that there was a progressive increase in the size of Amphidesma from the north to the south along this 30 mile extent or sandy beach. Similarly some numerical distribution according to size seemed evident between tide-marks, i.e. individuals or certain sizes and numbers appeared to occur at certain levels between low-water mark and high-water mark. In addition to this, over a period of months, populations or Amphidesma seemed to disappear and re-appear at various localities.

The sandy shore offered yet another challenge to the biologist and hence it was suggested that these earlier observations might be pursued further with particular regard to the instability of the environment. The sandy beach, then, would form a "natural laboratory" for what might be termed an exercise in physical zoology.

The objects of the present study, therefore, were: (a) to inquire into the numerical distribution and zonation of populations or Amphidesma along the shore of Pegasus Bay, (b) to acquire some know-ledge of the local growth rates, survival of larvae and consequent distribution or the various age groups of Amphidesma in relation to the physical factors on the sandy beach and in the offshore waters, (c) to investigate variations in external shape, i.e. form of shell, with respect to age, locality and species in .Amphidesma. It was hoped that this aspect of the study might reveal the limitations of the classification of molluscs based on the features of external morphology so often used in systematic conchology without regard to either changes in proportion with age or with general ecological conditions, and (d) to present a picture in general terms or the life history or the individual AplphidesM from the egg to the breeding adult with reference to the vicissitudes of life in the beach habitat.