53 



the coast of West Africa, the Canary, Madeira, and Azore Islands, 

 which are not to be obtained in Morocco or the soutla of the European 

 continent. 



In treating of the distribution of uiollusca, some notice should be 

 taken of species which are considered to be local, that is, limited to 

 a particular locality or a small ai'ea, though the number of these is 

 constantly diminishing as we extend our knowledge. 



A newly-discovered species is supposed to be confined to the spot 

 where it was first obtained, until it re-appears in a locality where it was 

 perhaps least expected to be met with ; this more particularly happens 

 with some of the deep water species. Many marine shells supposed to 

 be peculiar to the Canary Islands are probably common to a large un- 

 explored tract of the African coast — several of them I have found in 

 Madeira. 



The interesting ahell Pleurotoma, oxManrjelia ieres, was first discovered 

 by Professor E. Forbes on the coast of Lycia ; it was next met with, a 

 few years afterwards, in the Channel of the Minch, between the Isle of 

 Skyeand the outer Hebrides, and has since been procured pretty generally 

 throughout the British seas. I have obtained it in various parts of the 

 Mediterranean, in the Canary and Madeira Islands, and it is in Loven's 

 enumeration of the shells of Scandinavia. Crenella rhombea, one of the 

 rarest species of our seas, having been only found in three or four 

 instances and one locality (off Weymouth), I have met with in the Bay 

 of Gibraltar, Gulf of Tunis, and abundantly off Lancerote, one of the 

 Canary Islands. Chemnitzla fenestrata, discovered only six years ago 

 in Dartmouth harbour, has since been obtained from at least two other 

 localities in the South of England ; I have procured it in Vigo bay, and 

 it has been found m the result of dredgings from Alexandria in Egypt. 

 It would be easy to cite many similar instances, but what I have 

 mentioned may suffice to prove how unsafe it is to conclude that a species 

 is restricted to a peculiar locality, from the negative fact of its not haviiig 

 been found elsewhere. 



For my part, I believe that very few, if any, marine species are confined 

 to very small areas. With reference to the curious niollusk Bifrontia 

 znnclcca, which was only known as a very rare fossil obtained at Messina, 

 and no recent example even of the genus recorded to have been seen, 

 but which I found living in considerable abundance at Madeira, it is 

 probable that further researches will discover its existence in other parts 

 of the Atlantic ; if not, it will appear to be an exception, and that its 

 present habitat is its last refuge and stronghold, after becoming elsewhere 

 extinct. 



