The distribution of the sub-genera of Cochlostyla on the different islands of the Philippine group affords important evidence on the geological relation of the islands to one another. Thus we find Orthostylus and Hypselostyla occurring in the central islands and S. Luzon, but not in Mindanao or Mindoro; we find Chrysalis peculiar to Mindoro, Prochilus to Mindoro and the Cuyos, Ptychostyla to Luban, all these being sub-genera of very marked characteristics. Six out of the fifteen sub-genera are entirely absent from Mindanao, although occurring on the islands in the immediate vicinity. The little group Tablas-Romblon-Sibuyan are entirely deficient in certain sub-genera which occur on the islands surrounding them on all sides.[371]
Other forms peculiar to the Philippines are Diaphora, a section of Ennea with a curiously produced mouth, and several sub-genera of the Naninidae (Vitriniconus, Vitrinoidea, Hemitrichia). The great Rhysota here find their metropolis. Another very marked group of Helix is Obbina, 19 of the 25 known species being peculiar.
The Helicidae proper of the Philippines are still held in check, as in the greater part of the Indian region, by the Naninidae. The single Trachia and Plectotropis, and the 2 species each of Plectopylis and Satsuma, indicate affinities with Indo-China. Further important Indian relationships are seen in the great Nanina and Cyclophorus, which here attain almost Indian dimensions; in Kaliella (8 sp.), Sitala (2), Clausilia (1). Among the operculates we still have 1 Alycaeus and 1 Coptochilus. Singularly enough, several Indian genera which occur here are not found in the intervening islands of Borneo, Sumatra, or Java, e.g. Streptaxis, Hypselostoma, Ditropis, Acmella, and Cyathopoma. The curiously tubed Malay operculates, Opisthoporus, etc., fail to reach the Philippines proper, although occurring in Borneo and N. Celebes; one of them reaches Palawan. The strikingly Malay genus Amphidromus reaches Palawan, but no farther (1 sp.), while 2 species reach Mindanao, and one of these penetrates as far as Bohol and S. Leyte. Amongst the slugs, Mariaella occurs again only in the Seychelles, and Tennentia only in Ceylon.
Fig. 212.—Helix (Obbina) rota Brod., Philippines.
Land and Fresh-water Mollusca of the Philippines
| Streptaxis | 1 |
| Ennea | 10 |
| Mariaella | 3 |
| Tennentia | 1 |
| Helicarion | 21 |
| Vitrinopsis | 5 |
| Vitrinoidea | 1 |
| Rhysota | 17 |
| Trochonanina | 2 |
| Euplecta | 28 |
| Hemiplecta | 11 |
| Hemitrichia | 15 |
| Xesta | 2 |
| Macrochlamys | 5 |
| Microcystis | 3 |
| Lamprocystis | 17 |
| Bensonia | 4 |
| Vitriniconus | 16 |
| Sitala | 2 |
| Kaliella | 8 |
| Trochomorpha | 21 |
| Endodonta | 1 |
| Plectopylis | 3 |
| Plectotropis | 1 |
| Aulacospira | 3 |
| Pupisoma | 1 |
| Satsuma | 2 |
| Dorcasia | 2 |
| Chloritis | 7 |
| Obbina | 19 |
| Papuina | 1 |
| Phoenicobius | 7 |
| Cochlostyla | 247 |
| Amphidromus | 2 |
| Hapalus (?) | 4 |
| Hypselostoma | 1 |
| Pupa | 4 |
| Clausilia | 1 |
| Subulina | 3 |
| Prosopeas | 2 |
| Opeas | 4 |
| Geostilbia | 1 |
| Tornalellina | 1 |
| Succinea | 3 |
| Vaginula | 2 |
| Ancylus | 1 |
| Limnaea | 3 |
| Planorbis | 3 |
| Physa | 2 |
| Melania | 50 |
| Pirena | 2 |
| Bithynia | 1 |
| Vivipara | 7 |
| Ampullaria | 5 |
| Acmella | 2 |
| Diplommatina | 41 |
| Arinia | 6 |
| Pupina | 5 |
| Registoma | 7 |
| Hargreavesia | 1 |
| Callia | 2 |
| Pupinella | 3 |
| Helicomorpha | 4 |
| Coptochilus | 1 |
| Alycaeus | 1 |
| Leptopoma | 42 |
| Lagochilus | 11 |
| Cyclophorus | 31 |
| Ditropis | 7 |
| Cyathopoma | 5 |
| Cyclotus | 19 |
| Omphalotropis | 3 |
| Helicina | 18 |
| Georissa | 3 |
| Anodonta | 1 |
| Cyrena | 3 |
| Corbicula | 7 |
Islands adjacent to the Philippines.—The Philippines are connected with Borneo by two distinct ridges or banks of elevation, which enclose between them the Soo-loo or Mindoro Sea. There can be little doubt that these ridges represent the ancient highway of transit, by which Indo-Malay species passed into the Philippines. The depth of the sea on either side is profound, ranging from an average of about 1000 fathoms west of Palawan to 2550 off the south-west coast of Mindanao.
It appears that the fauna of the Soo-loo ridge is definitely Philippine up to and including Bongao, Sibutu, and Bilatan, the last islands at the Bornean end of the ridge. On these are found two species of Cochlostyla and an Obbina.
The Palawan ridge may also be described as more or less Philippine throughout. One species of Cochlostyla occurs on Balabac, just north of Borneo, and two on Palawan, but these are perhaps counterbalanced by the definitely Indo-Malay Amphidromus and Opisthoporus (1 sp. each). At the northern end of the ridge, on Busuanga and Calamian, the Philippine element predominates.