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 map (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, Bohol, 

 CebUjNegros, and Panay, not one of which is specially characterized by 

 any one particular group, but all of which have a great many groups 

 in common. The islands which would still remain isolated would 

 be Luban, Mindoro, the Tabias-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 sub- 

 merged 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 ^. There is no need to assume 

 that the elevation and submergence affected all the islands simul- 

 taneously, or that it has not been several times repeated. 



The conclusions that we arrive at by a study of the genus Cochlo- 

 styla are not illustrated, to any very considerable extent, by the 

 distribution of other genera of Land-MoUusca occurring in the 

 Philippines^. 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 ludo-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. 



^ The soundings in the Surigao Strait are, even in the most recent charts , 

 Tery infrequent, and the extent of shallow water between Mindanao, Leyte, and 

 Samar is probably exaggerated on the accompanying map. 



^ Mr. Everett writes to me as follows: — •" Oebu, Siquijor, part of Bohol, 

 almost aU 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." 



' E. g. Miplecta, Hemitrichia, Obhina, which are almost peculiar to the 

 group. 



