course to the different steps or periods of their life and growth, as they are marked out by the indented lips or foldings of the shell; untill they arrive at their full size; when they begin to fortify themselves, by bulwarks and strong holds, against the injuries and incidents, which attend old age. This is most conspicuous in the cowree, or concha Veneris of Lister, book iv. sect. 9.
Mr. Reaumur [1] found, by repeated experiments, that land snails form their shells by juxtaposition: as the animal grows in bulk, the shell is increased by a mucous matter emitted from the body of the animal, whic hhardens by degrees into a testaceous substance: and from the experiments upon land shells that great naturalistconcludes by analogy, that all testaceous bodies are formed in the like manner, particularly those of the turbinated kind.
To this general rule an objection is made by Mr. Poupart, from the formation of the cowree, or concha Veneris before mentioned: but this learned gentleman was not aware, that this shell is first 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 sawing one of them through the middle, I found a turbinated shell within the outer wall, consisting of six or seven convolutions, but no stages, or periods, of the indented lip appeared in any of the convolutions, as we find in the helmet shell, and several of the buccinæ. I then began to consider how this animal enlarged its dwelling; and was fully convinced, that
- ↑ See his book of insects.
