302 SPECIFIC GRAVITY OF SHELLS. 
Mr. De la Beelie has recently published a list 
of the specific gravities of living shells of different 
genera, from which he shews that their weight 
and strength are varied in accommodation to the 
habits and habitation of the animals by which 
they are respectively constructed ; and points out 
evidence of design, such as we discover, in all 
carefully conducted investigations of the works 
of nature, whether among the existing or extinct 
forms of the animal creation.* 
of the Ammonites and of many cognate genera of carnivorous 
Trachelipods, at the termination of the Secondary period, i. e. 
after the deposition of the Chalk formation. 
* " It can scarcely escape the observation of the reader, that, 
while the specific gravities of the land shells enumerated is 
generally greatest, the densities of the Jioating marine shells are 
much the smallest. The design of the difference is obvious : 
The land shells have to contend with all changes of climate, and 
to resist the action of the atmosphere, while, at the same time, 
they are thin for the purpose of easy transport, their density is 
therefore greatest. The Argonaut, Nautilus, and creatures of the 
like habits require as light shells as may be consistent with the 
requisite strength; the relative specific gravity of such shells is 
consequently small. The greatest observed density was that of 
a Helix, the smallest, that of an Argonaut. The shell of the 
lanthina, a floating Molluscous creature, is among the smallest 
densities. The specific gravity of all the land shells examined 
was greater than that of Carara marble ; in general more ap- 
proaching to Arragonite. The freshwater and marine shells, 
with the exception of the Argonaut, Nautilus, lanthina, Litho- 
domus, Haliotis, and great radiated crystalline Teredo from the 
East Indies, exceeded Carara marble in density. This marble 
and the Haliotis are of equal specific gravities." — De la Beche's 
Geological Researches, 1834, p. 76. 
