OCR Text |
Show 458 REV. A. H. COOKE ON [May 17, Conclusions with regard to the Development of Cochlostyla. It is probable that the distribution of Cochlostyla, above indicated, is due primarily to the union and separation of the various islands, perhaps more than once repeated. It can hardly be an accident which excludes Orthostylus, Hypselostyla, and Helicostyla from Mindanao, while Corasia and Calocochlea are not excluded, or which so sharply separates Mindoro from its near neighbour Luzon. The accompanying m a p (p. 455) shows that an elevation of the sea-bottom, of not more than 100 fathoms, would be sufficient to unite together all the great islands of Luzon, Leyte, Samar, Burias, Masbate, Bobol, Cebu,Negros, and Panay, not one of which is specially characterized by any one particular group, but all of which have a great m a n y groups in common. The islands which would still remain isolated would be Luban, Mindoro, the Tablas-Romblon-Sibuyan group, Siquijor, and, possibly, Mindanao, all of which are characterized by the prominent absence or presence of marked subgenera \ When we know that the whole of the adjacent island of Borneo has been submerged for a depth of probably twice this amount, the probability that similar oscillations of level have occurred in the Philippines is largely increased, and, as a matter of fact, comparatively fresh coralline limestone occurs in many places in the islands at a height of at least 2000 feet above the sea 2. There is no need to assume that the elevation and submergence affected all the islands simultaneously, or that it has not been several times repeated. The conclusions that we arrive at by a study of the genus Cochlostyla are not illustrated, to any very considerable extent, by the distribution of other genera of Land-Mollusca occurring in the Philippines3. The natural inference is, that the genus Cochlostyla, as a whole, is of comparatively recent development, dating, in all probability, from a time much posterior to the introduction of the bulk of the Iudo-Malay genera, and subsequent to the final separation of the group from Borneo and possibly from Celebes. The oldest of the subgenera appear to be Chlorcea, Corasia, and Calocochlea, which are universally distributed, being common alike to Luzon, Mindoro, Mindanao, and the central group. Orthostylus and Hypselostyla were probably developed in the central group after the final separation of Mindanao. Mindoro and Luban (the only possessors of peculiar subgenera) must have been isolated very early, although perhaps the union of Mindoro with the Cuyos Is. continued after the separation of the former from Luzon and from Panay. 1 The soundings in the Surigao Strait are, even in the most recent charts, very infrequent, and the extent of shallow water between Mindanao, Leyte, and Samar is probably exaggerated on the accompanying map. 2 Mr. Everett writes to me as follows:-" Cebu, Siquijor, part of Bo hoi, almost all Leyte, N.E. Mindanao, Tablas, Eomblon, and parts of Samar and Luzon are covered with thick caps of recent coral-limestone. There is a good deal of limestone in Palawan and the Calamianes. The island of Mindoro is . . . not overlaid (at least as seen from the sea) by recent coral-limestone, as so many of the Philippine group are." 3 E. g. Euplecta, Hemitrichia, Obbina, which are almost peculiar to the group. |