VOL. LIV.] PHILOSOPHICAL TRANSACTIONS. 93 



cidentally broken or decayed by time, when forsaken by their inhabitants. 

 For no testaceous body can be formed without an inhabitant ; nor does it appear 

 that any belemnite was ever formed without an alveolus, or concamerated shell. 



The conical cavity and its nucleus are always proportioned to the bulk of the 

 belemnite, but not to its length : some are 4 times longer in proportion to the 

 alveolus than others. The apex of the conical cavity, where the alveolus is first 

 formed, in some runs up about half the length of the whole belemnite ; in 

 others not a 6th part of the whole, (fig. 8) : but the aperture, or upper 

 chamber (fig. 13, b, c,) is equally proportionable to the bulk or circumference 

 of the belemnite, of whatever size or shape ; and is the seat* or dwelling-place 

 of the animal that forms the belemnite. In what manner this work is executed, 

 we shall now endeavour to explain. A considerable part of marine bodies, 

 especially those of the testaceous tribe, are generally buried in mud or sand, 

 except some few which stick to rocks, &c. as the limpets and periwincles ; by 

 which means we are prevented from making those remarks on the several stages 

 of their growth, which an accurate inquirer would desire. We must therefore 

 have recourse 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 ; till 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. Q. 



Mr. Reaumur -|- 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, which hardens by degrees 

 into a testaceous substance : and from the experiments on land shells that great 

 naturalist concludes by analogy, that all testaceous bodies are formed in like 

 manner, particularly those of the turbinated kind. To this general rule an ob- 

 jection 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 Mr. Piatt to examine into 

 the growth of the cowree ; and by sawing one of them through the middle, he 

 found a turbinated shell within the outer wall, consisting of 6 or 7 convolutions 

 but no stages, or periods, of the indented lip appeared in any of the convolu- 

 tions, as we find in the helmet shell, and several of the buccinae. He then 

 began to consider how this animal enlarged its dwelling ; and was fully convinced 



• Wc never find a belemnite with part of the alveolus, but the vestigia or marks of the remaia- 

 der appear in the cavity, and are continued to the verge of it.— Orig. 

 t See his book of insects. — -Orig. 



