part 4] OF THE MIOCENE OF CEYL05T. 589 



various dates from Burdigalian to possibly Pontian, with a balance 

 of opinion in favour of Vindobonian, and perhaps of Upper Vindo- 

 bonian (Tortonian). It is possible, of course, that distinct species 

 may be confused under this name ; but, although a variable form, 

 it has features that are clearly marked, and there is no recog- 

 nized species with which it can well be confused. It is not quite 

 unique in its geographical distribution. Suess (1885) long ago 

 pointed out that Plaenna miocenica was a distinctly Indo-Pacitic 

 element in the Miocene of Siwa, and Deshayes described from the 

 same series of beds as O. virleti in Greece a thick-shelled Pinna 

 which is possibly the same as that found with it in Ceylon. 

 Further, there seems to be an affinity between the species of 

 Spondylus associated with O. virleti in Egypt and Ceylon respec- 

 tively, and the same may be said, more doubtfully, of the species 

 of Strombits and JSfatica. 



It is noteworthy that in Burma and on the Mekran coast 

 O. virleti occurs among the latest marine Miocene deposits, which 

 have freshwater equivalents at no great distance, and are followed 

 by a great series of freshwater beds. In Ceylon, on the contrary, it 

 seems to mark the first transgression of the sea over a very ancient 

 land-area. This affords a very clear example of Prof. Hang's 

 principle that marine regression from the geosynclines is contem- 

 poraneous with marine transgression on the continental areas 

 (Haug, 1900). In the Mediterranean region, owing to the greater 

 complexity of the earth-movements, the contrast is not so obvious. 

 In Egypt the O. -virleti Beds transgress on to part of a continental 

 area depressed between tw T o series of faults ; elsewhere they usually 

 rest, unconformably, upon earlier Miocene. As Prof. Haug has 

 said, — 



" Le Tortonien correspond .... a une phase de retrait [dans ]es regions 

 alpines], car l'invasion de certaines parties de la region alpine par les eaux 

 du "2eme t'tage mediterranean " est due, non pas a une "transgression", ni 

 meme a une " ingression ", mais bien a une "irruption " de la mer, due a des 

 effondrements.' (1900, p. 707.) 



If, then. Ostrea virleti dates the moment when the sea was 

 beginning its invasion of continental areas while still lingering in 

 parts of the geosynclines, its circumstances gave the best oppor- 

 tunity for a possible short-lived shallow-water connexion between 

 the Indian Ocean and the Mediterranean, enabling this oyster and 

 a very few other species to migrate from the former to the latter. 

 This, at least, is the direction in which the other species spread, 

 and. tin nigh O. virleti is usually thoiight of as a Mediterranean 

 form, it must be remarked that the few recent species that offer 

 any suggestion of affinity to it are all Indo-Pacific forms, and a 

 closely allied species (O. vespertina Conrad) is found in the 

 Upper Miocene of California. 



The fundamental antithesis between the continental area of 

 Ceylon and the Himalayan-Malayan geosyncline will explain why 

 Orbiculina malabarica, which characterizes the Miocene of 

 Northern Ceylon and Southern India, should be unknown else- 



