IN GENERAL. 685 
or mantle of molluses. Ri&iaumuR? 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 
lamine 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 l’dccroissement des coquilles, Mém. de V Acad. royale des Sc. 
1709, Paris, 1733, pp. 364—400; Paris, 1741, pp. 303—311 ; comp. also Pot Testac. 
utriusque Sicilie, Tom, 1. (in the introduction) and HEUSINGER System der Histologie. 
Eisenach, 1823, 1, 2tes Heft, s. 236—242. 
