47 s 
CONCHOLOGY. 
lip 
Formation 
of hollow 
libs. 
Of the Con- part of the body bc-ing depoiited on the grooves, chan- 
Partsof ne * Sj 01 i ^ r * se ’ which were formed when the anterior 
Shells, Sec. P art the body occupied that part of the (hell, fills 
«— — J™™* them up completely, and leaves the lurface quite fmooth 
and polilhed. New additions being made to the fliell 
as the growth of the animal requires it, the fmooth 
furface of the body advances forward, and fills up with 
its fecretions what is now grooved ; while the new part 
of the fliell, which correfponds to that part of the body 
which has an unequal furface, only prefents this ap- 
pearance. It is in this way that the ribs or grooves 
are formed in different fpecies of oftrea, cardium, and 
other bivalve fhells. 
But there is a peculiarity of ftrudture in a fpecies of 
cockle, the white fluted or ribbed cockle, cardium 
cojlatum , which feems more difficult of explanation in 
its mode of formation. The ribs of this fpecies are not 
only of the ufual ftrudlure of other fpecies of ribbed or 
grooved (hells, but are particularly diftinguiffied by 
having them hollow. The whole number of the ribs 
amounts to about 1 8 on each valve, of which the 1 1 ex- 
terior ones are of a triangular form, of about three lines 
high, and hollowed through their whole length, from 
the beak to the margin of the valves. To have a di- 
ftinft notion of the formation of thefe hollow, triangu- 
lar ribs, it is neceffary to conceive, that the margin of 
the anterior part of the animal is deeply channelled or 
grooved ; and when this part of the body is in contact 
with the recent (hell, the tips or elevations are formed, 
and are then open to the internal furface of the (hell ; 
but the pofterior part of the body being hard and 
fmooth, never comes in contact with the excavated 
part of the ribs. On the contrary, as the teflaceous 
matter is excreted from this part of the body, it is de- 
poiited on that part of the internal furface of the (hell 
which it touches, ftretches acrofs the deep grooves, 
and forms the third and interior fide of the triangular 
tribs. 
Thus it appears, that fpines, tubercles, and all 
other protuberances on the furface of bivalve (hells, 
owe the peculiarity of their form and ffiape to the pe- 
culiar flrudture of different organs fituated on the ante- 
rior margin of the body of the animal, and are com- 
pofed of the teflaceous matter which is excreted by 
thefe organs. The nature of the procefs is the fame 
as in univalve (hells of a fpiral form. The diverfity 
only appears in the difference of the organs and ftruc- 
ture of the animals which inhabit different (hells. To 
a fimilar procefs may be aferibed the formation of Arise, 
of feales, and of various excavations which fometimes 
accompany them. 
Sect. V. Of the Production of Pearls . 
Pearifound In treating of the conftituent parts of (hells, it was 
In mother- obferved, that the compofition of the pearl appears, 
of-pearl from analyfis, to be precifely the fame as the mother- 
(hells. of-pearl, or thofe (hells in which the pearl is ulually 
found. From this we mud conclude, that the pearl, 
and the mother-of-pearl, are produced by the fameiecre- 
tion. It appears, from the obfervations of naturalifis, 
and indeed it might have been expected, from the fi- 
milarity of compofition, that all teflaceous animals, 
whofe fhells come under the defeription of mother-of- 
pearl, occafionally produce pearls. 
Chap. V. 
and of 
(trite, &c, 
Different opinions have been entertained with regard Of the Con- 
to the caufe of the formation of this precious produc- ^ lUient 
tion. According to fome, it is merely a morbid con- ghells^&c 
cretion, formed within fome part of the body of the ’ “■ 
animal, or at leaft within the fliell, without any ap- 
parent external injury j while others fuppofe that it is 
only owing to wounds which the (hell, or the animal, or 
both, have received from accidental caufes, or from the 
action of infedts, or fome teflaceous animal, making 
perforations in the fliell. It is not improbable that 
pearls may be formed in both ways. lrz 
Every day’s experience informs us, that fimilar con-Suppofed 
cretions are formed in different cavities of the bodies t0 ^ e mor " 
of other animals ; but without any obvious caufe or ex- 
ternal injury. The formation of fuch concretions, as, 
for inftance, biliary and urinary calculi, producing the 
mod excruciating diforders in the human body, are too 
fatally known. Thefe concretions, no doubt, owe 
their origin to the dileafed or unhealthy adlion of the 
veffels fecreting the fluids in which they are formed. 
By this difeafed adlion producing a fuperabundance of 
the matter which enters into the compofition of the 
concretion ; or this matter in the fluid date meeting 
with fome folid body, which becomes a nucleus, is at- 
tradted by it, and depofited in concentric layers, till 
the concretion acquires a larger or fmaller fize, ac- 
cording to the duration and quantity of the fecretion 
and depofition. In the fame way, it feems extremely 
probable the pearl may be frequently formed ; the 
matter of which it is compofed being conftantly fecreted 
by the animal for the produdlion of the new part of 
the (hell. If then this matter ftiould at any time be 
produced in greater quantity than what is neceffary to 
form the inner layers of the (hell, and particularly if it 
(hould meet with a folid particle of any body, it will 
be attradled by it, and thus conftitute the rudiments of 
a pearl, which will receive conftant additions of con- 
centric layers, and increafe in fize in proportion to the 
age of the animal and the quantity of matter depofited. 
Pearls, it is faid, have been found within the body of 
the animal. If this be true, the pearly matter, in its 
paffage through the veffels of the body, mull have met 
with fome nucleus, around which the concentric lay- 
ers have been formed. In moft cafes, however, the 
pearl is found loofe in the fliell, entirely detached from 
the animal. It mull then have been formed of the 
matter which was thrown out of the body ; but it is 
not unlikely that pearls are formed both ways, or that 
the fame pearl may be partly formed within the body 
of the animal, and be afterwards excluded, and arrive 
at its utmofl fize, while it remains loofe in the fliell. 
But, according to others, the pearl owes its forma- or formed 
tion to fome external injury. The following feems to from ex- 
be a pretty diftindt view of this opinion. When Fau-l ernal in " 
jas de St Fond vifited Loch Tay, he was led to make Jury ‘ 
fome inquiries concerning the pearl-fifhery, which had 
been carried on in feveral parts of the river Tay for 
fome years. Shells were brought to him ; and in thefe 
fhells the fiffiermen pretended to find pearls, which 
they expedled to fell at a higher rate, as they were 
found in the prefence of the traveller. But he informs 
us, that they attempted to impofe on him, by intro- 
ducing a pearl fecretly into the fhells as they opened 
them. Obferving this circumftance, he told them that 
he could know at once, by examining the outfide of 
i the 
