60 EOCENE MOLLUSCA. 
fresh-water deposits of the Eocene epoch, remains of numerous species belonging to 
various of the land and fresh-water genera, constituting this order, occur in abundance. 
These species are, for the most part, without any living analogue, but some few occur 
which appear to be identical with species still in existence. 
I believe that, as yet, remains of land inhabiting genera have not been found below 
the Eocene formations. It must not, however, be assumed from that circumstance that 
these latter forms of animal life date their existence with the Eocene Epoch. The older 
formations with which Geologists are at present acquainted, are, with the exception 
of the Wealden group and the Coal-measures, of marine origin; and the preservation 
in them of the remains of land-shells would be due only to accidental circumstances. 
Individuals might occasionally be transported by rivers or currents of water into 
estuaries, or be swept away by an irruption of the sea. To such a cause is to be 
attributed the presence of a specimen of Bulimus ellipticus, found by Mr. Wetherell in 
the London Clay at Primrose Hill; but the number deposited in the bed of the deep 
sea by the agency of such casualties, must necessarily be small, and it need not excite 
surprise that their remains have not occurred in the older formations. 
Sus-Orper—ADELOPNEUMONA (Gray), INOPERCULATA (Férussac). 
Sect. a. Terricoia, Fleming. 
GropuHita, Férussac. 
Family—HE.IciDz&. 
Genus 6th. Henix. Linn., 1758; Brug., 1792; Lam., 1801. 
Shell turbinated, orbicular, sub-globose, or depressed; spire more or less elevated, 
with several convex whorls, generally smooth; the last often large and ventricose ; 
aperture entire, transverse, oblique, lunate, or semi-ovate, impressed by the prominent 
part of the body whorl, and sometimes furnished with one or more lamelliform teeth ; 
peristome confluent with the columella, generally thickened internally, or having the 
edge reflected, especially on the side covering the umbilicus ; without an operculum. 
Notwithstanding that in Lamarck’s time but little was known of the comparative 
anatomy of the animals belonging to this family, we must concur in the regret 
expressed by M. Deshayes (2d edit., ‘Hist. Nat.,’ &c.) that that illustrious naturalist 
did not attempt a systematic arrangement of the numerous species forming this genus. 
A much more intimate knowledge of the anatomy of the animals has since 
been acquired; and it appears that, although the shells present a great variety of 
forms, differences of organisation of importance sufficient to justify the separation of 
genera to receive them, do not exist, or, at all events, have not been observed in the 
animals. This strong general resemblance extends, in fact, to the whole of the 
