MOLLUSCA. 
441 
J. Lewis speaks about the species of Melantho and other freshwater-shells 
in the valley of the Mohawk. Am. Journ. Conch, iv. pp. 2-4, 133-13G, and 
241-245. 
b. Marine Mollusca. 
1 . Seas of Europe, 
The dredging report among the Shetland Isles, by J. 
Gwyn Jeffreys, contains, besides a few additions to the British 
fauna, as Pleurotoma carmata (Bivona) and Montacuta donacina 
(S. Wood), a list of 75 species common to the North Sea and 
the Mediterranean ; many of them have been described as dis- 
tinct species ; and their identity with northern forms has been 
made out by the author during a recent visit to Italy. Both 
faunas are much more different with regard to their littoral 
species than to those which live in somewhat greater depths. 
The principal results of the au thorns observations from his 
dredging excursions arc summed up thus (Ann. & Mag. Nat. 
Hist. ii. pp. 298-305) : — 
1. There are only two principal zones concerning the bathymetrical distri- 
bution, the littoral and the submarine. 
2. Specimens or varieties of the same species are larger in the littoral and 
laminarian zones than in deeper water. 
3. The size of North-European specimens is usually greater than that of 
South-European specimens of the same species. 
4. The colour of specimens from the greatest depths is not less vivid than 
of those from shallow water, although each zone has colourless specimens. 
6. Mollusca inhabiting deep water have consequently a larger supply of 
oxygen for the aeration of their gills than those which live in shallow 
water. 
6. The occurrence of the same species in the North Sea and the Mediter- 
ranean results partly from former geological or cosmical conditions, and 
partly from a communication which once existed between the Bay of Biscay 
and the Gulf of Lyons. 
7. Exotic and oceanic shells are carried northwards b}" westerly winds, 
and not directly by the Gulf-stream, which does not reach our coasts. 
8. Land- and freshwater mollusca are scarce in Shetland, owing to the 
scantiness of succulent vegetation and of lime ; the specimens are also smaller 
than southern ones. 
9. Semifossil shells of Arctic species are met with on the sea-bottom at 
considerable depths, and at some distance from land : probably the sea-bed 
was formerly more elevated, these mollusks living in shallow water, and is, 
perhaps, still sinking. 
10. Species recorded from the Coralline Crag and earlier deposits, and sup- 
posed to be extinct, have now been discovered living in the Shetland seas. 
The statements contained in Nos. 1, 3, 4, and 7 are opposed 
by R. MacAndrew, from his own observations, in the same 
journal, pp. 357-362. 
Shetland Isles. 359 species of marine mollusks inhabiting 
