The leading genera are Euthria, Triton, Cominella, Bullia, Nassa, Cypraeovula, Oxystele, Fissurella, Fissurellidaea, Patella, and Chiton.
The Mollusca of Kerguelen Island and the Marion and Crozets groups show relationship partly with South America, partly with the Cape, and partly with South Australia and New Zealand, thus showing some trace of a circumpolar antarctic fauna corresponding to, but not nearly so well marked as that of the circumpolar arctic sub-region. Among the remarkable forms discovered off Kerguelen are Neobuccinum and a sub-genus of Struthiolaria (Perissodonta).
B. The Indo-Pacific Region
includes the whole of the coast-line of the Indian and western Pacific oceans, from about East London in South Africa to the north of Niphon (lat. 42°) in Japan, with the Red Sea and Persian Gulf, the whole of the Indo-Malay Archipelago, Polynesia to the Sandwich Islands in the north-east, and Easter Island in the south-east, and Australia to Swan River in the west, and to Sandy Cape and Lord Howe’s Island in the east. It is especially the region of coral reefs, which furnish so favourite a home of the Mollusca, and which are entirely absent from the Atlantic Region.
(1) The Indo-Pacific Sub-region proper (which includes the whole of this region except that part defined below as the Japanese Sub-region) is by far the richest in the world. The marine Mollusca of the Philippines alone (in some respects the nucleus of the whole region) have been estimated at between 5000 and 6000 species, and Jousseaume estimates Red Sea species at about 1000. Some prominent genera are very rich in species. Garrett enumerates from Polynesia 81 species of Conus, 60 of which occur on the Viti Is., 21 on the Sandwich Is., and only 14 on the Marquesas, where coral reefs are almost absent; 82 species of Cypraea, Viti Is. 44, Sandwich Is. 31, Marquesas only 13; 167 species of Mitra (besides 29 recorded by others), Viti Is. 120, Sandwich Is. 36, Marquesas 7. Of 50 existing species of Strombus, 39 occur in this region, and 10 out of 11 Eburna.
The following important genera are quite peculiar to the region: Nautilus, several forms of Purpuridae, e.g. Rapana, Magilus, Rapa, Melapium, and Ricinula; Tudicla, several forms of Strombidae, e.g. Rostellaria, Terebellum, Pteroceras, and Rimella; Cithara, Melo, Neritopsis, Stomatia, Malleus, Vulsella, Cucullaea, Tridacna, Hippopus, Libitina, Glaucomya, Anatina, Aspergillum, and many others.
The number of species common to the Red Sea and Mediterranean is exceedingly small, some authorities even denying the existence of a single common species. The present author, from an examination of the shells dredged by MacAndrew at Suez, regarded 17 species as common, and Mr. E. A. Smith has confirmed this view with regard to 8 of the species in question.[386] The Mollusca occurring in Post-pliocene beds at Suez show that Mediterranean species lived there in comparatively recent geological times.
The opening of the Suez Canal appears to have already induced several species to start on their travels from the Mediterranean to the Red Sea and vice versâ. Two Red Sea species (Mactra olorina Phil., Mytilus variabilis Kr.) had in 1882 established themselves at Port Said, while two Mediterranean species (Pholas dactylus L., Solen vagina L.) had reached Ismailia.[387]
(2) The Japanese Sub-region consists of the Japanese Islands to Niphon, together with Corea and a stretch of adjacent mainland coast of unknown extent. The warm Kuro Siwo current, sweeping up between Luzon and Formosa, permits tropical species to extend much farther north than on the opposite shores of America, where a cold polar current keeps them back. A certain number of species, however, are common to the two shores of the Pacific, and a few circumpolar species occurring on our own coasts reach Japan, e.g.Trophon clathratus, Puncturella noachina, Mya arenaria, Modiola modiolus, Lasaea rubra, and Nucula tenuis.
Among the characteristic genera are Fusus, Siphonalia, Columbarium, Hemifusus, Rapana, Chlorostoma, Pleurotomaria, Haliotis, and Cyclina.