DISTRIBUTION OF ONCHIDIUM. 375 



found in North Australia and the western portion of the 

 ' Pacific, in the whole of the Chinese Seas as far as Japan, in the 

 Indian Ocean and the Malayan Archipelago, and on the east 

 coast of Africa. The Onchidia with dorsal eyes bave precisely 

 the same distribution. 



Only one exception existed until lately. On the coast of 

 .Congo, where, from Giinther's catalogue of recent fishes, Peri- 

 ophthalmus has long been known to exist, no Onchidium had 

 been found, and even in the latest list of African land-mollusca, 

 which we owe to the industry of Martens, no Onchidium is in- 

 cluded. However, in answer to an inquiry from me, I was 

 informed by that admirable malacologist that he was in posses- 

 sion of several species which had lately been brought back from 

 Congo by the German expedition to that coast. He was good 

 enough to send mo a specimen to examine; but unfortunately 

 it reached me in so bad a condition that it was impossible to 

 arrive at any positive conclusion with regard to the presence or 

 absence of dorsal eyes. 



It therefore is possible that the Onchidium found on the 

 West African coast may not have such eyes, and thereby a 

 strong argument would be raised against the hypothesis I have 

 put forward. For, according to that view, we should be inclined 

 to regard the dorsal eyes as an organ indispensable to the genus, 

 since by them only could its extirpation by the fish be pre- 

 vented. However, there are spots where the fishes which we 

 regard as the chief enemies of the Onchidium do not live, and 

 where nevertheless the molluscs occur and are by no means 

 rare. One species, long since described by Cuvier as Onchidium 

 celticum, is found on the Atlantic shore of England and France ; 

 another occurs in America, on the high northern coast ; others 

 again live on the west coast of both North and South America ; 

 the Galapagos Islands have their peculiar species, and in the 

 eastern parts of the Pacific, as in New Zealand and Australia, 

 many species are found. In all these places the fishes are 

 absent, and all the species of Onchidium here mentioned — 

 almost all of which I have examined anatomically — are devoid 

 of dorsal eyes, and at the same time of the glands which, acting 

 as weapons, can alone serve to demonstrate the use of the dorsal 



