THE ANNALS 
AND 
MAGAZINE OF NATURAL HISTORY. 
XXVI.—Zo0-Geological Considerations on the Freshwater 
Mollusca. By Epwarp Forsss, Esq., M.W.S., For. Sec. 
B.S., &c. 
Tue 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. Planordis 
presents the same phznomena, 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. & Mag. Nat. Hist. Dec. 1840. R 
