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 Bulimies 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. 



Sub-Order— ADELOPNEUMONA (Gray), INOPERCULATA (Ferussac). 



Sect. a. Terricola, Fleming. 

 Geophila, Ferussac. 



Family — H e li c i d,e . 



Genus 6th. Helix. 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 



