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Show 1892.] THE LAND-MOLLUSCA OF THE PHILIPPINES. 459 Relations of the Philippines to the neighbouring Islands. The Philippines are connected with Borneo, and through Borneo with Java, Sumatra, and the mainland of S.E. Asia, by two distinct ridges or banks of elevation, which enclose between them the Soo-loo or Mindoro Sea. The first, or westernmost, of these, which stretches from a point S.W. of Mindoro to the northern Cape of Borneo, consists of the islands of Busuanga, Calamian, and Limi-capan, of the great island Palawan or Paragua, and the smaller islands Balabac, Balambangan, and Banguey. The entire length of this ridge is somewhat over 400 miles, not including the channel (about 50 miles wide at its narrowest point) between Busuanga and Mindoro. Of this, about 350 miles is land, and about 50 miles water of less than 50 fathoms in depth. The easternmost bridge, which stretches from Zamboanga, the extreme western point of Mindanao, to the N.E. corner of Borneo, consists of a continuous chain of small islands, the Basilan group, and the Soo-loo Archipelago. This ridge is only about 225 miles in length, but the largest island of the chain is scarcely 40 miles long, as compared with Palawan, which is over 250. O n either side of both ridges the depth of the sea is profound. A deep submarine valley1, with soundings of 670 fathoms to 1200 fathoms (the so-called ' Palawan passage '), runs in a N.E. and S.W. direction immediately west of and parallel to Palawan. The Soo-loo Sea is still deeper, soundings of 2225 fathoms and 2550 fathoms having been obtained off the S.W. coast of Mindanao, while profounder depths still have been fathomed in the Celebes Sea. A curious point about these ridges is, that a chasm occurs in each of them, and in each of them at one end, but not at the same end in both. The Palawan ridge is interrupted at its extreme northern end, between Busuanga and Mindoro, by a channel 50 miles broad and about 600 fathoms in depth (the Mindoro Strait). The Soo-loo ridge is interrupted at its extreme southern end by a channel only about 20 miles in width, but in parts over 500 fathoms in depth (the Silutu passage). Were it not for these channels, a rise of 100 fathoms in elevation of the sea-bottom would make a double direct communication by land between the Philippines and Borneo. There can be no doubt that Indo-Malay species of Mollusca have penetrated into the Philippines, in very early times, by both these ridges. Thus we find abundant in the Philippines the great Naninee and Cyclophori so characteristic of the larger Sunda Islands. Four2 1 It is important to notice this, since the ' Palawan passage' might be expected to mean the strait between Palawan and Borneo, whereas it means the fairway between Palawan and the dangerous ground to the west. Occasionally we find ' Palawan passage' given by inexact writers as a locality for Land- Mollusca, which is much as if ' Mozambique Channel' were given as a locality for a Madagascar Cyclostoma, or ' Bass' Strait' for a Tasmanian Helix. Pfeiffer (Mon. Hel. iv. 362) gives' Palawan passage' for his Bulimus trailli, and Tenison- Woods (Proc. Linn. Soc. N. S. Wales, ser. 2, iii. p. 1003) gives the same locality for Camcena trailli and palawanica. 2 Amphidromus jay anus, Lea, is probably not Philippine, and certainly not a Cochlostyla. Godwin-Austen records it (as a Cochlostyla) from Borneo (P. Z. S. 1891, p. 45). |
