IX GENERAL. 685 



or mantle of molluscs. REAUMUR l has illustrated the formation by 

 his experiments. He found, on boring the shells of living snails, the 

 aperture to become closed again by a thin layer occupying the whole 

 of it, to which other layers were afterwards added. Thus there was 

 no calcareous matter secreted at the edge of the aperture by vessels 

 running in the shell, as in the reparation of bones where the forma- 

 tion of new osseous matter proceeds from the extremities of the 

 fractured bones. When, however, REAUMUR supposes that the forma- 

 tion of shells is a mechanical transudation, that the secretion of lime 

 on the upper surface of the mollusc may be compared to calcareous 

 incrustations, which in certain waters and springs are formed round 

 bodies immersed in them, that the skin of the animal, like a sieve, 

 permits an adhesive fluid loaded with calcareous particles to escape, 

 and that this fluid by evaporation and rest loses its watery particles, 

 then his representation bears too forcibly the character of off-hand 

 ideas respecting living beings, and POLI is so far justified in calling 

 the origin of shells an organic origin, and in rejecting this mechani- 

 cal explanation. It is clear that shells by the addition of new 

 laminse become thicker. In a shell several layers or calcareous 

 scales lie upon each other, which in bivalves originate from the 

 point; hence the shell is, at this part, of greatest thickness, and 

 becomes gradually thinner towards the margin. Thus every shell 

 consists, as it were, of many others, all of which becoming larger 

 and larger, lie under each other, whilst the innermost, the last 

 formed, extends beyond the others at the margin. In oyster-shells 

 and many other bivalves this may be clearly seen, and snail-shells 

 also indicate the same; in younger univalves the number of turns is 

 fewer; the larger shells of the same species present a greater num- 

 ber of wreaths than the smaller, without however those wreaths, 

 that were already present in the young ones, increasing in size. So 

 also the spines, tubercles, and other excrescences of univalves are 

 at first short and obtuse, and become larger and more acute by the 

 addition of new layers. The increase however is not at all times 

 uniform, but in the cold of winter and the great drought of summer 



1 De la, formation et de I'dccroissement des coquilles, M&n. de I'Acad. royale des Sc. 

 1709, Paris, 1733, pp. 364 400; Paris, 1741, pp. 303 311 ; comp. also POLI Testae, 

 utriiwque Sicilies, Tom. I. (in the introduction) and HEUSINGEB System der Histologie. 

 Eisenach, 1823, I. -ztes Heft, s. 236 242. 



