MOLLUSCS. 771 



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, Nautilus 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 . 



1 Interesting contributions to the knowledge of the geographical distribution of 

 molluscs have been made in the last few years by the investigations of SABS, FORBES, 

 D'ORBIGNY, PHILIPPI and others. Compare A. D'ORBIGNY RecJiercJies sur les lois qui 

 president a la distribution geographique des Mollusques cdtiers marins, Ann. des Sc. not. 

 30 Sdrie, Tom. in. 1845, PP- *93 221 > E. FORBES On the topographical distribution 

 of marine Molluscs, Edirib. Phil. Journ. New Ser. April, 1844, and Ann. des Sc. not. 

 30 Ser. Tom. IV. pp. 117, 118, and LOV^N'S remarks on the geographical distribution 

 of Scandinavian Marine Molluscs, Ofversigt af Kongl. Vetensk. Alcademiens Forhand- 

 lingar. 1846, pp. 252274. 



492 



