I3IVALVIA. I 1 



Fimbria, Thetis, Cultellus, and Limopsis} In the higher group of Mollusca as many as twelve 

 genera may be enumerated, viz., Nautilus, Pterocera, Seraphs, Harpa, Metula* 

 Borsonia, Botclla, Pyramidella, Niso, Nematura, P/iorus, and Parmapltorus, 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, 1 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 Eocene 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 arc 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, &c, 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. 



1 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 iu the North Seas. These 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. 



