SUMMARY.1A critical ist of the species of Nudibranchiata previously recorded from the Red Sea is given.2A critical list of the species of Nudibranchiata previously recorded from the Mediterranean Sea is given.3A detailed comparison of the contents of these two lists shows clearly that there is a deep‐seated and fundamental difference between them. Indeed, nowhere in the world do seas so close together present such a striking divergence. The Red Sea is to be regarded as a sub‐area of the Indo‐Pacific and the Mediterranean a sub‐area of the North Atlantic region.4The specimens in the collection include four species of Tectibranchiata and fifteen species of Nudibranchiata. With one exception, all the Opisthobranchs collected are comparatively rare or new, so that it has been possible to add to the previous knowledge of a number of them. Of the four Tectibranchs,Philine vaillantiwas described incompletely from a single specimen in 1865 and no details given of the radula. The present is the second specimen and fortunately from the type‐locality, so that it has been possible to provide a more adequate description.Berthella oblongawas very briefly described in 1825, but not since, and no account was then given of the radula or labial armature. It has been possible to remedy this deficiency and refer the species definitely to its genus.Berthella saidensisis apparently a new species. Fifteen species of Nudibranchs were determined, and two of them found sufficiently striking to be placed in new genera.Coryphellina rubrolineatais a form in some ways allied toCoryphella, but unique among the æolids in the arrangement of the cerata and the character of the rhinophores.Bornellopsis kabretianalies near the curious genusBornella, but is distinctly differentiated therefrom by the peculiar form and relationship of the cerata and branchiae and in the shape of the rhinophore sheath.Casella foxiis a new species belonging to a small and interesting genus in some respects intermediate between Dorids and Glossodorids.Bœolidia moebiiis a little‐known member of a monotypic genus described originally from a single specimen from Mauritius and not previously recorded from the Red Sea.Plocamopherus ocellatuswas described originally from the Red Sea, but, in spite of the fact that it is the genotype, its radula has not been figured before.Archidoris vayssièreais again only the second specimen, and the previous one was in poor condition and only partially described. A number of details have been added, and it has been shown that it is notArchidoris stamineawith which it was first identified. Lastly,Kentrodoris labialiscomes from the type‐locality, and only a brief description with no illustrations has been given previously.5Thirteen species were found within the Canal. Of these, 10 have immigrated from the Red Sea and 3 are new species which have probably also come in from the south. Table 40 shows the distribution of all species found within the Canal. None have certainly immigrated from the Mediterranean. In addition to the above, 4 species (Philine vaillanti, Hallaxa decorata, Kentrodoris labialis, andGlossodoris runcinata) were found at the Suez mouth of the Canal and one new species (Berthella saidensis) at Port Said.6One marine Pulmonate,Peronia peronii, w