C 43 1 
courfe to the different fteps or periods of their life 
and growth, as they are marked out by the indented, 
lips or foldings of the fhell ; untill they arrive at 
their full fizej when they begin to fortify them- 
felves, by bulwarks and ftrong holds, againft the 
injuries and incidents, which attend old age. 
This is mofl confpicuous in the cowree, or concha 
Veneris of Lifter, book iv. fecft. 9. 
Mr. Reaumur [»] found, by repeated experiments, 
that land fnails form their fhells by juxtapofition : as 
the animal grows in bulk, the fhell is increafed by 
a mucous matter emitted from the body of the animal, 
whic hbardens by degrees into a teftaceoas fubftance : 
and from the experiments upon land fhells that 
great naturaliftconcludes by analogy, that all teftaceous 
bodies are formed in the like manner, particularly thofe 
of the turbinated kind. 
To this general rule an objection is made by Mr. 
Poupart, from the formation of the cowree, or con- 
cha Veneris before mentioned: but this learned gen- 
tleman was not aware, that this fhell is firft a buccinum, 
forming many convolutions before it draws in the 
verge to form the indented lip. 
It was this very objection of Mr. Poupart, which 
led me to examine into the growth of the cowree ; 
and by fawing one of them through the middle, Ifound 
a turbinated flaell within the outer wall, confiding of 
fix or feven convolutions, but no ftages, or periods, 
of the indented lip appeared in any of the convolu- 
tions, as we find in the helmet fhell, andfeveral of the 
buccinae. I then began to confider how this animal 
enlarged its d welling ; and was fully convinced, that 
[«] See his book ofinfedts, 
G 2 
no 
