Fig. 206.—Cyclophorus siamensis Sowb., Siam.
Land operculates are very richly developed. In all, there are 17 genera and 104 species known. The tubed operculates attain their maximum, and Cyclophorus is even more abundant than in India. Fresh-water bivalves abound. Dipsas and Pseudodon are common to China, and Unio and Anodonta are profusely represented. A curious resemblance to S. America appears in this group, a single Mycetopus occurring, the only species not Brazilian, while Arconaia appears very closely to approach the Hyria of the same locality. Several genera of the Hydrobia type (Pachydrobia, Jullienia, Chlorostracia) are peculiar.
Land and Fresh-water Mollusca of the Siamese Province
| Streptaxis | 20 |
| Ennea | 3 |
| Helicarion | 7 |
| Microcystis | 3 |
| Sesara (?) | 1 |
| Medyla | 1 |
| Xesta | 4 |
| Macrochlamys | 6 |
| Kaliella | 5 |
| Hyalinia (?) | 1 |
| Hemiplecta | 14 |
| Rhysota | 2 |
| Trochomorpha | 6 |
| Trochomorphoides | 3 |
| Plectopylis | 5 |
| Stegodera | 2 |
| Plectotropis | 12 |
| Trachia | 3 |
| Fruticicola | 2 |
| Acusta | 2 |
| Chloritis | 8 |
| Dorcasia | 1 |
| Camaena | 5 |
| Hadra | 5 |
| Obbina | 1 |
| Amphidromus | 33 |
| Bocourtia | 2 |
| Buliminus | 4 |
| Hypselostoma | 2 |
| Tonkinia | 1 |
| Clausilia | 15 |
| Opeas | 7 |
| Spiraxis (?) | 2 |
| Subulina | 1 |
| Succinea | 4 |
| Vaginula | 7 |
| Limnaea | 7 |
| Planorbis | 6 |
| Canidia | 13 |
| Melania | 39 |
| Faunus | 1 |
| Bithynia | 9 |
| Wattebledia | 1 |
| Stenothyra | 4 |
| Hydrobia | 1 |
| Pachydrobia | 9 |
| Jullienia | 6 |
| Lacunopsis | 6 |
| Chlorostracia | 4 |
| Vivipara | 39 |
| Valvata | 1 |
| Ampullaria | 15 |
| Assiminea | 7 |
| Procyclotus | 6 |
| Dasytherium | 2 |
| Opisthoporus | 5 |
| Rhiostoma | 7 |
| Myxostoma | 1 |
| Pterocyclus | 7 |
| Cyclophorus | 28 |
| Leptopoma | 10 |
| Lagochilus | 6 |
| Pupina | 8 |
| Hybocystis | 3 |
| Alycaeus | 6 |
| Cataulus (?) | 1 |
| Diplommatina | 2 |
| Helicina | 4 |
| Georissa | 2 |
| Modiola (f. w.) | 2 |
| Dreissensia | 3 |
| Anodonta | 17 |
| Mycetopus | 1 |
| Pseudodon | 18 |
| Dipsas | 4 |
| Unio | 64 |
| Arconaia | 1 |
| Cyrena | 6 |
| Batissa | 1 |
| Corbicula | 35 |
(c) The Malay Province includes the peninsula of Malacca south of Tenasserim, and the series of islands beginning with Sumatra and stretching eastward up to the Ké Is., besides Borneo and Celebes. The Philippines form a separate province.
The Malay province is singularly poor in representative forms, whether we regard it as a whole or consider the islands separately. Not a single genus, with the exception of Rhodina (Malacca), appears to be peculiar. The contrast with the West Indies is in this respect very striking. Java, for instance, which is well explored, and almost exactly eleven times the size of Jamaica, has about 100 species of land Mollusca, while Jamaica has about 460.
This want of individuality in the land Mollusca of the Malay islands is accounted for by a consideration of the sea depths which separate them from the Asiatic mainland. The accompanying map, the red line on which is intended to show what would be the result of an elevation of the sea bottom for no greater amount than 100 fathoms, exhibits clearly the fact that these islands are practically a part of Asia, a large stretch of very shallow sea extending between Siam and the greater part of the north-west coast of Borneo.
In all probability the three great islands of Sumatra, Java, and Borneo were united with the mainland of Asia, and with one another, at a period, geologically speaking, comparatively recent. This follows from the general uniformity of their land Mollusca, both as regards one another and as regards the mainland. Nor do the smaller members of the island series—Bali, Lombok, Sumbawa, Flores, Timor, and Timor Laut—show any marked individuality in the possession of peculiar genera. Wallace’s line is absolutely non-existent, so far as the land Mollusca are concerned. The really noticeable break in distribution comes with the Aru Is., for while the Tenimber group (Timor Laut, etc.) are decidedly Malay, and the Ké Is., in the poverty of our information, uncertain, the Aru Is. are as Papuan as New Guinea itself. The profound depths of the Banda Sea to the north, and the Timor Sea to the south, appear to have kept the islands from Flores to Timor Laut free from the intrusion of any Moluccan or any considerable Australian element. The Moluccas, as has been already remarked, besides possessing considerable peculiarities of their own, unite a mixture of the Malay and Papuan elements, and serve as a sort of debatable ground for the meeting of the two.
