Organs employed in the Classification of the Mollusca. 387 



2. From Behring's Straits, Greenland, Florida, Brazil, Cape 



Horn. 



3. „ Oregon, California, Peru, Cape Horn. 



4. „ Kamtschatka, Japan, China, Tas- 



mania. 



5. Cape, Arabia, India, Sumatra, Tasmania. 



The three first-named coast-lines may zoologically be con- 

 sidered the boundaries of the Atlantic Ocean, in the same way 

 that America is considered an island in the Atlantic Ocean, 

 separated from the Pacific Ocean by the western edges of the 

 polar streams and the great abyss between the Galapagos Islands 

 and the Sandwich Islands. The coast-lines are divided into 

 analogous faunae according to the climate: 1. Polar; 2. Sub- 

 polar (North Sea) ; 3. Subtropical (Mediterranean) ; 4. Tropical 

 (Guinea). The southern polar and subpolar are not sufficiently 

 known (Kerguelen Island, St. Paul's, &c). 



It would seem easy for most animals, even for Mollusca, the 

 young of which generally are natatory, to move or to be carried 

 by currents from one end of these coast-lines to the other; but 

 in reality this' is not the case. The temperature of the sea 

 seems to put a nearly insurmountable barrier to the distribution 

 or migration of species, and even of genera. A general view 

 of those faunae shows that they are composed of animals of all 

 subkingdoms, classes, orders, families of the animal kingdom 

 (Gulls, Seals, &c). The genera are often different, although 

 allied ; and the species are nearly always different in each fauna. 

 The question now arises, Are these species and genera originally 

 different, or changed by various physical causes, such as climate, 

 light, the saltness of the sea, &c? The influence of these causes 

 on the form and size of shells is chiefly seen by comparing 

 the faunae of the Baltic and Mediterranean with that of the 

 ocean. 



It would be a very slow process to compare the species of all 

 these faunae, chiefly because no museum possesses a sufficient 

 collection of animals in spirits for investigation of the teeth, 

 even of European species. The specific importance of the teeth 

 is evident, chiefly from the researches of Troschel ; many species 

 which were considered slight varieties are now proved to be 

 distinct — e. g., Natica millepunctata and N. maculosa, Natica 

 clausa and N. consolidata, &c, &c, Viviparus contectus and V. 

 fasciatus, Bithinia ventricosa and B. tentaculata. 



The genus Aporrhais, which was abundant in former geo- 

 logical periods, from the Oolitic formation upwards, is now 

 restricted to five or six recent species inhabiting the Atlantic 

 only. Aporrhais occidentalis is found on the coast of Canada 



26* 



