PTEROPODA. GASTEROPODA. 417 
erit, in the neighbourhood of Hastings, Tilgate Forest, 
Horsham, and other places in the Weald of Sussex, abounds 
in casts of the same species, associated with the Uniones, 
previously described. In the cliffs on the southern shores 
of the Isle of Wight where the Wealden beds emerge, and 
also in the Isle of Purbeck, these shells are equally abun- 
dant. Together with the Uniones, they occasionally appear 
in the limestone, called Sussex Marble; and in the Isle of 
Purbeck there are beds of limestone wholly composed ot 
bivalves belonging to these two genera, and presenting, in 
polished slabs, markings formed by sections of the enclosed 
shells. 
FOSSIL PTEROPODA. 
Tn the Ludlow strata there are found small fragile elon- 
gated conical shells without chambers, which are supposed 
by Professor E. Forbes to be identical with a recent genus 
of pteropodous mollusca, common in the Mediterranean, 
called Creseis. They seldom exceed two inches in length. 
Of another genus, named Conularia, six species have been 
discovered in the Silurian formation.* 
FOSSIL SHELLS OF GASTEROPODA. 
The univalve shells, as we have previously explained, are 
the calcareous cases, or coverings, of a more highly organ- 
ized class of molluscous animals, than the inhabitants of the 
bivalves (see p. 366), for they possess a head and mouth 
with jaws, eyes, and feelers; and while the Acephala, with 
but few exceptions, are incapable of locomotion, the Lnce- 
phala are almost all of them furnished with organs of pro- 
gression, and can creep, climb, and swim, or float on the 
surface of the water. Their shells are for the most part 
formed of one valve, hence the name of wnivalve; but in 
* See Geol. Trans. second series, vol. vi. p. 325. 
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