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Not AvailableIT is evident from the valuable reports on the fauna and flora of the Gulf of Mannar, brought out in 1903-1906 by Herdman and others as also from the contributions of Thomas (1884), Thurston (1894), Hornell (1922), Gravely (1927) and Pearson et al. (1929), to mention only a few, that this area is remarkable for its richness and variety of fauna and that the inshore sea-bottom in the Gulf forms an ideal habitat for the growth of the shell fishes which sustain a good fishery. The records of observations made along Ceylon coast during the latter part of 1904 by Hornell (vide Herdman, 1905) revealed that considerable changes took place periodically in regard to the biological and physical conditions of the sea-bottom there. As for the Indian side many years have lapsed since Hornell (1922) gave his first evaluation of the faunistic conditions and idea of the habitat of the economically important molluscs of the Gulf of Mannar. It was not known since then whether any notable changes had taken place, in the length of time that had elapsed, here also like its counterpart off the Ceylon coast and if so to what extent. An opportunity arose in 1961 when the F.A.O. sponsored a collaborative scheme for conducting direct underwater observations using SCUBA (Photographs 1 & 3) to study the area between 10-27 metres depth off Tuticorin. In addition to carrying out the above collaborative work assigned, we were able to conduct some biological observations of our own on the fauna of the rocky bottom, managing to get some extra time on many days after the main work was over. Many of the interesting observations made by us have been serialized and published already and in this last paper in the series some more observations are recorded and the impressions gained by us are presented so that it would serve as a guide-line for future workers. Detailed report of the main work of underwater survey done in collaboration does not fall within the purview of this paper and should be expected elsewhere at a later date.Not Availabl