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 dis- 

 tinct 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, Calaraian, and Lirai- 

 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. 



On either side of both ridges the depth of the sea is profound. A 

 deep submarine valley\ 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 S&itu 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 MoUusca have 

 penetrated into the Philippines, in very early times, by both these 

 ridges. Thus we find abundant in the Philippines the great Nanince 

 and Cyclophori so characteristic of the larger Sunda Islands. Four ^ 



1 It is important to notice this, since the ' Palawan passage ' might be ex- 

 pected 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- 

 MoUusca, which is much as if ' Mozambique Channel ' were given as a locality 

 for a Madagascar Cyclostoma, or ' Bass' Strait ' for a Tasraanian 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. 



^ Amphidromusjayanus, Lea, is probably not Philippine, and certainly not 

 a Cochlostyla. Gt-odwin-Austen records it (as a Cochlostyla) from Borneo 

 (P. Z. S. 1891, p. 45). 



