466 CONCH 
Of the Con- Some fpecies of patella, which were brought from 
Parteof ^ ac ^ e ‘ ra ) were a ' 1 " 0 fubjefled to chemical examination, 
Shells &c by the fame philofopher. When expofed to a red heat in 
G-— 1 a crucible, they emitted a perceptible fmell of horn or 
feathers •, and by farther examination, by fblution, the 
proportion of carbonic matter depoiited appeared to 
be greater, and the proportion of carbonate ot lime lefs, 
than what was indicated by the refult of the experi- 
ments on porcellaneous (hells. When unburnt fhells 
belonging to the fame fpecies, were immerfed in nitric 
acid very much diluted, the epidermis feparated, and 
the whole of the carbonate of lime was diffolved. A 
gelatinous fubflance, nearly in a liquid (late, remain- 
ed, but it did not retain the figure of the fhell, and ex- 
hibited no appearance of a fibrous ftrudlure. Thefe 
fhells, therefore, contain a larger portion of gelatinous 
matter than the porcellaneous fhells, but the other corn- 
er, ponent part confiils entirely of carbonate of lime. 
Mother of- Shells compofed of mother -of -pear l . — Shells of this 
peari. defeription were fubjedled to fimilar experiments with 
the former. When the common oyfler was expofed to 
a red heat, the effects were the fame as thofe which 
were produced by the fame procefs on the fpecies of 
patella from Madeira. The folution of the unburnt 
fhell was alfo fimilar, excepting only that the gelatin- 
ous part was of a greater confiltency. When the river 
muffel was burnt in a crucible, it emitted much fmoke, 
with a ilrong fmell of hurnt horn or cartilage ; the 
fhell became of a dark gray colour, and exfoliated. By 
folution in the acids, the proportion of carbonic matter 
ffeparated was greater, and that of carbonate of lime 
obtained was lefs, than from the other fhells on which 
experiments were made. 
When an unburnt fhell of this defeription was im- 
merfed in diluted nitric acid, a rapid folution and effer- 
vefcence took place ; and at the end of two days, the 
whole of the carbonate of lime was nearly diffolved. 
A feries of membranes now only remained, of which 
the epidermis conflituted the firfl. Thefe membranes 
ilill retained the figure of the fhell. The carbonate 
of lime was at firfl readily diffolved, becaufe the acid 
came eafily in contadl with it j but the procefs became 
flower, as it w r as more difficult for the acid to infinuate 
itfelf between the different membranes of which the 
lliell is compofed. The haliotis iris, and the turbo 
olearius, were found to refemble this muffel, except 
that the membranaceous parts were more compact and 
denfe. 
When thefe fhells are deprived by an acid of the 
carbonate of lime, which gives them their hardnefs, 
they appear to be formed of different membranes, ap- 
plied llratum fuper ftratum. Each membrane is fur- 
nifhed with a correfponding coat or cruit of carbonate 
of lime, and it is fo fituated, that it is always between 
every two membranes, beginning with the epidermis, 
and ending with the internal membrane, which has 
been laft formed. The animals which inhabit thefe 
flratified fhells, increaie their habitation by the addi- 
tion of a flratum of carbonate of lime, which is fe- 
cured by a new membi'ane. And as every additional 
llratum exceeds in extent that which was previoufly 
formed, the fhell becomes flronger in proportion as it 
is enlarged ; and thus the grow'th and age of the ani- 
mal may be denoted by the number of flrata of which 
the fhell is compofed. Similar experiments were made 
o L O G Y. Chap. Vo 
on pieces of mother-of-pearl as they are imported from Of theCcn- 
China, and with precifely the fame refults. They ap- ft ltuent 
peared to be compofed of the fame gelatinous matter shefis^&c 
and carbonate of lime. In all the fhells of this deferip- < . 
tion which were immerfed in acids, the membranaceous 
parts retained the exadl figure of the fhell, and they 
appeared diftindlly to be compofed of fibres, arranged 
in a parallel direction, correfponding to the configura- 
tion of the fhell. ?0 
Pearl . — The conflituent parts of pearl appear to be Pearl, 
fimilar to thofe of mother-of-pearl. They are compofed 
of concentric coats of membrane and carbonate of lime, 
and refemble in flrudlure the globular, calcareous con- 
cretions which are known by the name of pfolithes. 
The iridefcence and undulated appearance of pearl and 
mother-of-pearl, evidently depend on their lamellated 
flrudlure and femitranfparency. 
From thefe experiments it appears, that fhells are Component 
compofed of carbonate of lime and gluten. In fofne, asP a ‘t-- 
in the porcellaneous fhells, the proportion of carbonate 
of lime is great, while that of the animal matter is • 
fmall ; and thefe may be regarded as the beginning of 
the feries \ while fhells that come under the defeription 
of mother-of-pearl are to be placed at the other extre- 
mity, having a fmaller proportion of carbonate of lime, 
and a greater proportion of membranaceous fubflance. 
In the firil the carbonate of lime is merely cemented by 
the animal matter ; in the latter the carbonate of lime 
ferves to harden the membranaceous fubflance. But 
between thefe two extremes, in the proportion of carbo- 
nate of lime and animal gluten, of which all tellaceous 
fubftances are compofed, there are no doubt numerous 
intermediate gradations, ariling from the nature of the 
animal to which they form a covering, its peculiar ha- 
bits, or mode of life. 
Sect. II. Of the Formation of Shells. 
The fhell or covering of tellaceous animals, has 
been confidered as in fome meafure analogous to the 
bones of other animals, although its formation and 
growth are very different, fince it ferves as a bafe or 
fupport to the mufcles, which are attached to its inter- 
nal furface. The principal ufe of the fhell, however, 
is to ferve as a covering or defence to the animal. ? 
Tellaceous animals are not only extremely different Teftaceous 
in external form, but alfo in the mode of their produc- animals 
tion. Some are viviparous, as the moil of thofe which oviparous 
inhabit bivalve fhells multivalves, and even fome of ViV1 P a * 
the univalves ; while the others, which form the far 
greater proportion, are oviparous. In one point, how- 
ever, they all agree, that whatever be the mode of 
production, whether from an egg, or diredlly from the 
uterus of the mother, the fhell is formed on the body of 
the young animal, and is proportioned to its bulk. 
The befl obfervations which have yet been made, and Reaumur’s, 
the moft elaborate inveiligation which has hitherto ap- inveltiga- 
peared, concerning the formation and deveiopement ofd° n> 
fhells, are thofe of the celebrated Reaumur, which were 
publilhed in the Memoirs of the Academy of Sciences 
for the year 1 709. The fame fubjefl has been profecut- 
ed by other authors, but their refults have been nearly 
the fame as thofe of this diflinguifhed naturalifl. Klein 
is almolt the only author who has advanced a different 
opinion. In his differtation concerning the formation 
