SPECIFIC GRAVITY OF SHELLS. 229 



The design of the Creator seems at all times to have been, 

 to fill the waters of the seas, and cover the surface of the 

 earth with the greatest possible amount of organized beings 

 enjoying life; and the same expedient of adapting the vege- 

 table kingdom to become the basis of the life of animals, 

 and of multiplying largely the amount of animal existence 

 by the addition of Carnivora to the Herbivora, appears to 

 have prevailed from the first commencement of organic life 

 unto the present hour. 



Mr. De la Beche has recently published a list of the spe- 

 cific gravities of living shells of different genera, from which 

 he shows that their weight and strength are varied in ac- 

 commodation 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.* 



lopods of the Transition and Secondary strata were furnished with an oper- 

 culum, as if to protect them against the carnivorous Cephalopods which 

 then prevailed abundantly; but that in the Tertiary formations, numerous 

 herbivorous genera appear, which are not furnished with opercula, as if no 

 longer requiring the protection of such a shield, aftei- the extinction 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, vvlille the 

 specific gravities of the land shells enumerated are generally greatest, the 

 densities of the Jloating marine shells are mucli the smallest. l"he 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 tlie 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 strengtii; 

 the relative specific gravity of such shells is consequently small. The 

 greatest observed density was that of a Helix, the smallest, that of an Argo- 

 naut. The shell of the lanthlna, 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 approaching to Ar- 

 ragonite. The fresh-water and marine shells, with the exception of the 

 VOL, I. — 20 



