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
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