MOLLUSCS. V4 
the number of species decreases, just as that of land animals does 
the greater the height is above the surface of the sea. Amongst 
the Cephalopods that have a shell, and which occur sparingly in 
the present creation, whilst in the previous world they were very 
numerous and different in form, Nawté/us is confined entirely to the 
Indian and South Seas. Of the molluscs that live on land, no 
genus is more generally diffused than Helix, of which some 
hundreds of species are known, and which occurs in all regions. 
The species of molluscs are in a greater degree than those of 
bivalves confined to limited regions, and thus, on the whole, differ 
more in different countries}. 
1 Interesting contributions to the knowledge of the geographical distribution of 
molluscs have been made in the last few years by the investigations of Sars, ForBEs, 
D’OrBIGNY, PHILIPPI and others. Compare A. D’OrBIany Recherches sur les lois qui 
président & la distribution géographique des Mollusques cétiers marins, Ann. des Sc. nat. 
3e Série, Tom, m1. 1845, pp. 193221; E. Forses On the topographical distribution 
of marine Molluscs, Edinb. Phil. Journ. New Ser. April, 1844, and Ann. des Se. nat. 
3e Sér. Tom. Iv. pp. 117, 118, and Lovén’s remarks on the geographical distribution 
of Scandinavian Marine Molluscs, Ofversigt af Kongl. Vetensk. Akademiens Férhand- 
lingar. 1846, pp. 252—274. 
49—2 
