20 
STKUOTURE OF SHELL. 
and its shell, which is also, as truly, an organ of the animal, confirms 
and supports the importance attached to the shell in classification. 
Shells are termed E.xternal, when they are capable of containing all 
or part of the animal ; and Intern.\l, when enclo.sed or concealed within 
the mantle, and serve not only as a support, but also as a iirotection 
to the internal and vital organs, and may he considered to represent 
Fir,. 11. — An Internnl Shell. 
Li > 11 . IX »iaxi»ins L. X 2, 
C'hrislcluirch, Hants., 
Collected hy Mr. C. Ashford. 
Fig. 13. — An External Shell. 
Helix noiioralis L., 
llltton, near Hath. 
Collected hy Miss F. M. Helc. 
the internal skeleton of tlie Vertebrates, giving similar evidence of 
their natural affinities and relationship, the contour and structure of 
the shell lieing always in harmony witli the organization of the aaiimal 
and with their differences in form and composition, establish how ex- 
ceedingly important is the study of the shell itself, and to geologists 
esjiecially this importance is vastly increased, as their fossilized re- 
mains are the only relics of the vast numbers of now totally extinct 
sjiecies, and from these remains alone, we are able by induction and 
comjiarison with onr recent forms, to infer with great jirobability the 
organization, habits, and habitats of these long extinct organisms and 
gain some clue to the physical conditions of these remote times. 
Though differing considerably in appearance, structure and texture, 
shells principally consist of an organic chitinous substance and car- 
bonate of lime, with a chemical formnla of CaCO.-j. M. Delacroix 
has stated that the shell of lleli.v jwmatln is composed of (idTb per 
cent, of carbonate of lime, KJ'dO i)er cent, ot other mineral substances, 
and 1S ()4 per cent, of organic matter, bnt a specimen of the same 
species from Cheltenham, examined by Mr. Crowther, yielded 
strikingly <lifferent results: the total weight of the shell was ()'2!)<S 
grams, which analysis showed to be composed of (j'CiO.') grams of 
inorganic substances and 'ITTb gram of organic matter, or 2’<S1S 
per cent, of organic substances and DT'DSIG per cent, of caihonate 
of lime and other earthy salts. An example of Jauukcu stiCfjnal/s, 
weighing '081 gram, also examined by Mr. Crowther, yielded on 
analysis ‘GGOd gram of mineral matter, chiefly carbonate of lime 
