490 THE MEDALS OF CREATION. Cuap. XI. 
in a fossil state the shells alone remain to afford any clue 
as to the structure and economy of the originals, characters 
have been sought for, by which the fluviatile or marine 
nature, and the carnivorous or herbivorous habits of the 
living mollusca may be determined. Asa general rule, it 
will be found, that the shells of terrestrial and fresh-water 
Gasteropoda have the aperture entire, as in the Garden Snail, 
and in the fossil shell, Zagn. 133, jig. 1; and that a large 
proportion of the marine species have the opening notched 
or channelled, as in the Whelk, and Lign. 133, figs. 3,4; 
and most of the species with entire apertures are herbivo- 
rous. But these inferences must be regarded in a very 
general sense, and it will require corroborative evidence to 
establish the marine or fresh-water nature of those fossil 
shells which do not bear a close analogy to known living 
species. * 
The various conditions in which the remains of univalve 
shells occur in the mineral kingdom have already been so 
fully explained, that but a few additional remarks on that 
subject are required (see p. 382). 
The Gasteropoda are found to progressively diminish in 
number with the antiquity of the deposits, and it was once 
supposed that this type of molluscous organization was not 
contemporaneous with the ancient Cephalopoda. My dis- 
covery of several genera associated with Ammonites in the 
chalk (see Foss. South D. pl. xviii, xix) first tended to 
invalidate this hypothesis; and the subsequent researches of 
Dr. Fitton, Professor Phillips, and other geologists have 
shown that the presence or absence of Gasteropoda in a 
stratum may generally be ascribed to the circumstance of 
a shell-less species.”—Prof. E. Forbes, Edin. Philos. Journal, vol. 
XXXVI. p. 326. 
The well known Tiger Cowry (Cyprea tigris) in its earliest stage has 
a minute helicoid (snail-like) shell. 
* See Ly. p. 30. 
