Fig. 208.—A, Opisthostoma Cookei E. A. Smith, Borneo; B, Opisthostoma grandispinosum G.-A., Borneo. Both × 8.
Amphidromus is a genus characteristic of the great Sunda Islands, attaining its maximum in Java (12 sp.). The Indian Glessula still has one species each in Sumatra, Java, and Borneo. One species of Streptaxis[369] occurs in Malacca, but Ennea (3 sp.) reaches as far east as Borneo and the Philippines. Parmarion, Helicarion, Ariophanta, and other groups of the Naninidae are well represented. Hemiplecta and Xesta are abundant and large, while the Rhysota of Borneo contain some huge sinistral forms. Rhodina is a remarkable form from Malacca, whose exact generic position is not yet settled. Clausilia has a few species on all the islands, the last occurring on Ternate, and a single Papuina (Moluccas and N. Guinea) occurs in Borneo.
The Island of Celebes marks the beginning of a distinct decrease in the Indo-Malay element. The Naninidae lose ground, in proportion to the Helicidae, Macrochlamys, for instance, being represented by only one species, and Hemiplecta by four. Other characteristic genera of the Indian region dwindle, such as Amphidromus, Clausilia, the tubed operculates, and Cyclophorus, while Sitala, Kaliella, Glessula, and Plectotropis disappear altogether. Comparing the total numbers of Naninidae and Helicidae from Sumatra to New Guinea, we obtain this interesting result:—
| Sumatra | Java | Borneo | Celebes | Moluccas | N. Guinea | |
| Nanina (all genera) | 26 | 32 | 51 | 22 | 36 | 40 |
| Helix (all genera) | 7 | 11 | 13 | 14 | 55 | 91 |
It will be noticed that the proportion of Naninidae to Helicidae, which has been nearly 4 to 1 in Sumatra, falls to 3 to 1 in Java, and rises again to 4 to 1 in Borneo (showing the essentially continental character of the island); in Celebes it further falls to 3 to 2, while in the Moluccas the scale turns and Helix has the advantage by about 8 to 5, and in N. Guinea by more than 2 to 1.
Fig. 209.—Amphidromus perversus L., Java.
There is the same absence of marked features of individuality in Celebes as in the islands dealt with above. Not a single genus is peculiar. The nature of the sea bottom between Borneo and Celebes, with its indications of a somewhat broad bridge over an otherwise deep channel of separation, would seem to account for and suggest the true explanation of the facts as they stand. At the same time, there are indications of a certain amount of contrast between N. and S. Celebes. The Indian element, which constitutes the preponderating majority of the fauna, is common to north and south alike. But the north part of the island, in which Obba and Obbina occur, shows decided relationship with the Philippines, while the occurrence of three Chloritis and one Planispira tend to approximate S. Celebes rather with the Moluccas.
The islands eastward of Java, from Bali to Timor Laut and the Tenimber Is., present no trace of individual peculiarities; they simply carry on the Indo-Malay fauna as though along a great peninsula. Even Timor, surrounded as it is on all sides by sea of profound depth, shows no sign of possessing even one peculiar genus. Amphidromus, perhaps the most characteristic of all Indo-Malay genera, occurs throughout, diminishing in numbers as we go eastward (Bali, Lombok, and Sumbawa 4 sp., Timor 2 sp., Timor Laut 1 sp.), while Plectotropis reaches no farther than Flores and Timor. The tubed operculates are altogether wanting. In Timor Laut we have Moluccan influence appearing in 3 Chloritis, and there is one (supposed) Corasia. Two Helices of a marked Australian type (Rhagada) occur, one in Flores, the other on Dama I., south-west of Timor. The configuration of the sea bottom (see map) would lead us to believe that the north-west coast of Australia once stretched a good deal nearer to these islands.
The Moluccas, taken as a whole, constitute a transition region between the Indo-Malay and the Papuan faunas, uniting, to a very considerable extent, the features of both. They fall into two well-defined groups. The northern, or Ternate group, consists of Gilolo (Halmahera), Batchian, and the outlying islands as far south as and including Obi major. The southern, or Amboyna group, consists of Buru, Ceram, Amboyna, and the chain of islands to the south-east of Ceram, as far as, and including the Ké Is.