No evidence of a definitely pathological nature was found during the course of these studies, although certain findings indicate that there are factors which may be considered as characteristic of mussels, and other similar bivalves, which are highly toxic. The quantities of poison present in the digestive diverticula of a single mussel were so small that its demonstration by histo-chemical methods was impossible. The observations have shown, also, that the mussels are not affected physiologically by the ingestion of this poisonous substance which is so highly toxic to warm blooded animals. However, these findings are taken from observations which have been made in a restricted area, and may be considerably in error if applied to another. Therefore, further studies must be carried on elsewhere and the results compared with these before conclusions may be drawn with any degree of certainty. A direct relationship between the quantity and nature of the stored food material in the digestive diverticula and the seasonal variations in the plankton has been definitely established.