BIVALVIA. I] 
Fimbria, Thetis, Cultellus, and Limopsis.’ In the higher group of Mollusca as many as twelve 
genera may be enumerated, viz., Nautilus, Péerocera, Seraphs, Harpa, Metula,’ 
Borsonia, Rotella, Pyramidella, Niso, Nematura, Phorus, and Parmaphorus, also now 
confined to the south-eastern seas, and these genera appear to represent a relationship to 
the Marine Fauna of the Eocene period more especially than any others I can instance. 
The land and fresh-water species have, I think, on the contrary, retired in an opposite 
direction, as their connexion appears to be more especially with the existing types of the 
American Fauna. Mr, Edwards has described fifty-seven species of Pulmonata from our 
Eocene Deposits, of which only ten, he considers to be identical with fossil species of the 
European Continent, and this I imagine arises from a difference in the direction of the 
rivers which flowed into the Paris Basin Sea, and which were quite distinct from the one 
that is presumed to have been emptied into the Hampshire Beds. 
The Bivalves now to be described comprise species of fresh-water animals as well 
as those from estuarine and marine deposits of the great Hocene Formation in this country. 
The estuarine animals were, no doubt, shallow-water species, and the principal part of the 
marine from the littoral or sub-littoral zones ; the deep-water portion of the period being, 
in all probability, the great Nummulitic deposit. We are able, in some degree, to surmise 
the probable depth of the sea of a marine deposit, from the collective indications of the 
various genera it contains, whose habits are presumed to be similar to those of existing 
analogues, but even in these suppositions extreme caution ought to be observed. The 
dredgings of Mr. M‘Andrew and others, among existing Molluscs present us with anomalies, 
which show that in some genera, as, for example, in Chiton, Trochus, &e., which generally 
inhabit the littoral zone, species have presented marked exceptions to this rule, as they 
have been found alive in deep water only. A conclusion, therefore, drawn from a single 
extinct species can not be entirely depended upon, inasmuch as the habits of the 
animal might have resembled those of the exceptional cases in existing genera. 
' This genus has one species living in the Red Sea, one at Singapore, one off the Cape of Good Hope, 
and one (perhaps two) have been found in the North Seas. hese last are probably the prolonged ex- 
istence of the Crag species ; otherwise the generic relationship is with the south and east. 
2 Mr. P. Carpenter has, in his valuable ‘Report on the Mollusca of the West Coast of North America,’ 
introduced four species, but the generic name is accompanied by a mark of doubt. 
