DISTEIBUTION OF MOLLtJSCA GENEEALLT. 259 



satisfactory. The notion of absolute limitation, which was enter- 

 tained when the provinces were first instituted, sees its own dis- 

 proof in the records of almost every new exploration, and the 

 annihilation of the very essentials which were considered requisite 

 for the framing of zoogeographical boundaries. The more phil- 

 osophical interpretation of the nature of species, and the more 

 general recognition of the fact that certain forms considered to be 

 peculiar to a definite region or district may also occur elsewhere, or 

 where they are assumed not to belong, even if they show no varia- 

 tion in their cha/racters, have done much to render the generally ac- 

 cepted provinces illusorjf. It is true that many of the regions now 

 recognised by conchologists are strictly defined as such, but it is 

 equally true, using the generally accepted criterion in the formation 

 of provinces — the inclusion of a certain proportion of peculiar 

 species — that new provinces, with entirely different boundaries, 

 and with as much claim to faunal peculiarity, might be instituted 

 in place of others that are also fully recognised. Until greater 

 harmony is reached by malacologists in their realisation of species, 

 and more regard paid to the facts of nature rather than to precon- 

 ceived notions, any attempt at delimitation of zoogeographical 

 boimdaries must prove at best only half satisfactory. 



The MoUusca proper — Lamellibranchiata, Gasteropoda, Ptero- 

 poda, and Cephalopoda — of which there are some 30,000, or more, 

 recent species, have a world-wide distribution, being found in 

 almost all parts of the earth's surface that have thus far been vis- 

 ited by man. From beyond the eighty-second parallel of north 

 latitude to the Equator, and from the surface of tUe sea to a depth 

 of sixteen or seventeen thousand feet, and to an equal height above 

 it on the land, they are everywhere more or less abundant. Despite 

 the extraordinary range which certain species are reputed to enjoy, 

 there is yet scarcely a single one that is in any way entitled to the 

 claim of cosmopolitanism, nor, indeed, any, if we except possibly 

 some pelagic species (pteropods, cephalopods), and the members of 

 the Arctic and Antarctic faunas, which occupy a complete circum- 

 ferential zone of the globe.* 



* A further exception may possibly have to be made in favour of some of 

 the abyssal species, concerning whose distribution we know practically noth- 

 ing. Mytilus edulis and Saxicava aretica are perhaps the most strictly cos- 

 mopolitan species known. 



