OCR Text |
Show 454 REV. A. H. C O O K E O N [May 17, channel, contains representatives of no subgenera which do not occur in Luzon. Only two subgenera (Corasia and Calocochlea) appear to be found there. Luzon.-The great size of this island, exceeding as it does in area all the other islands, excepting Mindanao, taken together, and its wide extension to the S. and E., cause it, besides developing a rich fauna of its own, to receive immigrants from subgenera not indigenous to it. Thus there is no group which is not represented on Luzon, with the exception of those peculiar to Mindoro and Luban, but, on the other hand., there is no group (except Pfeifferia) peculiar to it. It is characterized by a rich development of the four subgenera Corasia, Calocochlea, Helicostyla, and Orthostylus; its neighbourhood to Marinduque gives it its 2 species of Phengus, to the central group its 2 species of Hypselostyla. Catanduanes, Polillo, and Alabat, the three islands on its eastern side, the fauna of which is well known, present no peculiar features; the channels separating them from Luzon are shallow, and they are practically a part of the main island. Marinduque.-This island, although in other respects closely related to Luzon, stands out distinct from it in several respects, and is by no means so closely related to it as the islands just mentioned. The channel separating it from Luzon is deep, and apparently only just fails to exceed 100 fathoms in depth throughout its length. It is the metropolis of the subg. Phengus, 7 out of the 9 known species being found there. No species of Calocochlea, so abundant in Luzon, appears to occur. Orthostylus is abundant, and Hypselostyla is represented by one species. Leyte and Samar.-These two islands, which are separated from one another by a very narrow and shallow channel, are closely related. The San Bernardino channel, which separates Samar from Luzon, is not of great depth, and accordingly no subgenus occurs on these two islands which does not also occur on Luzon. On the other hand, the Surigao passage, which separates Leyte from Mindanao, is, at least in part, of considerable depth, and we find accordingly that two out of the three subgenera hitherto recorded from Leyte do not occur in Mindanao. Neither of the islands can be said to be well explored. Only Calocochlea and Orthostylus are recorded from Samar, and the same two, together with Hypselostyla, from Leyte. Burias and Masbate.-The subgenus Canistrum, so far as our information goes, appears to be peculiar to these two islands. This is the more strange, because the channel separating them from Luzon and from Samar is of no great depth, and the stretch of sea between Masbate and the N. coasts of Negros and Cebu is the largest piece of shallow water in the Philippines, scarcely exceeding 30 fathoms at any point. Eastward of Burias and Masbate the depths are considerable, and completely separate these islands from the Tablas- Romblon-Sibuyan group. Ticao does not appear to have been explored. Orthostylus is abundant on Masbate. Tablas, Romblon, Sibuyan.-These three islands, which are separated by very deep water from all their neighbours, are closely |