THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



XXVI. — Zoo-Geological Considerations on the Freshwater 

 Mollusca. By Edward Forbes, Esq., M.W.S., For. Sec. 

 B.S., &c. 



The Mollusca inhabiting fresh water are all testaceous ; such 

 as are univalve are either pulmoniferous or pectinibranchous 

 Gasteropoda ; such as are bivalve are Acephala Lamelli- 

 branchia. The consideration of the effect of climatal influence 

 on their generic and specific variations of form, and of the 

 comparative geographical distribution of the existing species, 

 leads to some conclusions which appear to bear importantly 

 on certain points in geology. 



The genera of Freshwater Pulmonifera exhibit few subge- 

 neric groupings of species, and those few are not climatally 

 centralized. Thus, the forms of Limneus are common to the 

 whole world, and the distribution of species is proportionably 

 extensive. The species of Limneus present near resemblances 

 whether gathered in England, in India, in Australia or in 

 America — they are often even specifically identical. Planorbis 

 presents the same phenomena, and the variations of form in 

 Physa can scarcely be regarded as exceptional. So also An- 

 cylus. Nor are the two characters most subject to the climatal 

 influence, those of size and colour, much affected by it, either 

 as regards the species of the genera or the individuals of the 

 species. Some of the largest forms of Limneus and Planorbis 

 are northern, and in them colour never varies climatally. The 

 negative influences which appear to affect the number of spe- 

 cies as we go northwards are rather structural than climatal. 



Not so however with the pectinibranchous Gasteropoda 

 inhabiting fresh water. Among them we find the number of 

 genera and of species increasing as we go south, and peculiar 

 forms characterizing warm countries. A Paludina or Mela- 

 nia from the warmer regions of our earth has an eye-character 



Ann. fy Mag. Nat. Hist. Dec. 1840. r 



