Sottas—On the Origin of Freshwater Faunas. 99 
Littorina is a marine genus; but it contains two sub-genera, Lithoglyphus 
(S. E. Europe, and Central Africa, LZ. tanganyika) and Limnotrochus (L. tanganyika), 
which inhabit fresh water. 
Compared with the rich variety of forms which they present in other parts of 
the world, the freshwater Gasteropoda are but poorly represented in our British 
streams. The chief genera are Limnea, Ancylus, Physa, Planorbis, and Valvata, 
belonging to the euthyneurous Gasteropoda, and Paludina and Bithynia, belonging 
to the Streptoneura. 
Although not quite germane to our subject, we may stop to point out the 
remarkable thinness of the shell in these molluscs, and its frequent correlation with 
a thick epidermis (periostracum). The thinness of shell is suggestive at first sight 
either of a deficiency of carbonate of lime in fresh water, or of greater difficulty in 
extricating it from a solution in which sodium chloride is scarcely present. 
Analyses, however, show that fresh water is usually by no means deficient as com- 
pared with sea water in calcium carbonate; and the fact that the Unionide frequently 
secrete quite massive shells indicates that the absence of sodium chloride has no 
appreciable effect. The erosion of the umbones of Unio shells and earlier-formed 
whorls in Paludinz seems to suggest an excess of free carbonic acid, and perhaps 
other acids which might tend to hinder the secretion of solid shell: and possibly 
the thick epidermis not only represents layers which in a marine shell would be 
calcified, but also functionally serves to protect the already formed shell from solu- 
tion.* On the whole it seems most probable that the thin shell has arisen by 
natural selection, and is correlated with the lower specific gravity of fresh water as 
compared to salt. This diminished density would render it needful for the free 
movements of the animal that it should be disburdened of all unnecessary weight. 
On the other hand, the thicker shells of marine mollusca, so often found broken, are 
probably correlated with the occurrence of powerful shifting currents and storm 
waves in the medium which they inhabit. 
Passing on to our main inquiry, we find that the euthyneurous Gasteropoda 
usually deposit their eggs in jelly, which is attached to some foreign object, and 
the young emerge in a fully formed state. In the streptoneurous division we have 
Bithynia and Paludina: the former attaches its eggs, but the latter—and this is a 
significant fact—is viviparous, the young molluscs leaving the mantle chamber of 
the parent as young adults. 
The Crustacea furnish us with abundance of freshwater forms, Copepods, 
Branchiopods, and Ostrapods, Isopods and Amphipods; and Palzemonine, and 
Astacine among the Decapods. But the first three groups contain none but minute 
forms, which are capable of distribution by birds and probably winds. They 
* Semper has already suggested that the epidermis serves this purpose. 
+ Some interesting remarks bearing on this subject will be found in a Paper by Mr. A. R. Hunt, in 
the Proceedings of the Royal Society, London, 1882, p. 8 (reprint). 
