l G 
CONCHOLOGY. 
According- to this opinion of Knorr, the (hell increafcs 
by addition or aggregation ; but it is more confonant to 
the funple operations of nature to fuppofe, that it is by 
extenfion that the (hell takes the (ize adapted to the fpe- 
cies, as well as to the growth of the animal. There is 
certainly a fyftem of arteries, as in all folid parts and 
bones of animals, conjoined in the (hell, by which the 
nourifhing moifture paffes to or from the inhabitant; and, 
according to this gcnerical formation, every fyftem of ar¬ 
teries, with its particular organs, conforms to the (frac¬ 
ture and wants, of the included animal. 
As to the beautiful defigns and colours of (hells, Knorr 
proceeds to explain them on the principles of animal 
fluids. He fays that a matter flows from the animal into 
the (hell, of a confiftency like foam ; different, at times, 
in the fame animal, according to the difference of the 
particular humours, and organical refervoirs ; juft as in 
other creatures, where the blood is red, the bile green, 
the urine yellow, the chyle white, &c. Now, if the or¬ 
ganical refervoirs, and the fmall veins, which ramify 
thence near to the furface of the (hell, are difpofed in cir¬ 
cles, lines, or figures, the moifture being of another co¬ 
lour, cannot prefent itfelf on the furface but in the fame 
colour. This moifture being hardened and augmented 
by continual addition through the l'paces of the (hell, 
and thus more diffolved, and as it were brought to per- 
feffion, it muft be that the (ketch or outline of the (hell 
will (hew the true difpofition of the fibres, veins. See. 
though only of a hair’s .breadth, ahd a Ho the pores. It 
cannot appear improbable that this (hould be the true 
conftrufUoh of thel’e creatures, becaufe we fee different 
ftriated and fpeckled fnails, with and w ithout (hells ; and 
alfo (imilar lines and decorations in a great many fpecies 
of caterpillars. Hence, as the colours fpring from the 
reflection of the rays of light, perpetually made on the 
plates of the furface, and which arife from the different 
diffolutions of the fmalleft particles, this author does not 
hefitate to attribute the colours of (bells to the (Irudlure 
of their organical fecretories. And as every animal is 
fnbjeff to certain difeales, which can change and alter 
the colour of their humours, and alfo by the functions of 
cjigeition, diffolution, fecretion, See. fo without doubt 
fea animals are fubjeiii to the fame mutabilities of nature, 
which thus become the caufes of their great variety of 
colours. Thofe who, in order to explain the formation 
and growth of (hell-fifh, fuppofe a fyftem o( arteries, lay 
that the liquors which flow from the animal into the (hell, 
although of one and the fame colour, can, by the petri¬ 
fication that takes place fuccefiively in the extremities 
of the fmalleft veins towards the exterior furface, take 
different colours; juft as the fame nourifhing juices of the 
human body can be differently coloured by the mixtures 
and Accretions. The above reafoning is no lefs applica¬ 
ble to figures and paintings, or to fmall variations of 
ftruifture; for the body or fibres of an animal may be 
badly formed ; it may have the pores ftraight and large, 
fo that it cannot failto produce a difference in the exter¬ 
nal appearance of the (hell, which muft not on that ac¬ 
count be taken for a different or fubordinate fpecies. 
This remark feems the more neceffaiy, in order that fuch 
things might not contribute to incrsale the genera and 
fpecies of (hells unneceilarily, in a Jyftematicai diviiion. 
From a bare calculation made by Knorr, the data of 
which he formed from the diver (fries of the colours of 
thofe fhells he had only in his own poffiefiion, he makes it 
appear that, there.would be two thoufand different (hells, 
without counting the fpecies which muft; be buried in the 
bottom of the lea, and which we know nothing of but 
by the pytrifications, which prove to us their exiftence. 
M. de Reaumur appears to have given a fatisfaftorv 
account of the formation of the (hell of the garden fnail, 
founded on a courfe of very ingenious experiments, re¬ 
lated in the Paris Memoirs, He there fupports the. theory 
of Knorr, by endeavouring to (how, that this fubftance 
Is produced merely by the perdurable matter of,the ani*. 
mal condenfmg and afterwards hardening on irs furface, 
and accordingly taking the figure of its body, which has 
performed the office of a mould to it; in (liort, that the 
(hell of a fnail, and, as he fuppofes, of all other animals, 
poffeffed of (hells, was only the produft of a vifeous tranf- 
udation from the body of the animal, containing earthy 
particles united by mere juxtapofition. 
But it was M. Heriflant, in the Memoirs of the Academy 
of Sciences for 1766, who firft difeovered the itrudfure of 
fhells to be organical. In the numerous experiments, that he 
made on an immenfe number, and a very great variety, of 
animal (hells, he conftantly found that they were compofed 
of two diftindV fubftances j one of which is a cretaceous 
or earthy matter, and the other appeared, from many ex¬ 
periments made upon it by burning, diftillation, or other- 
wife, to be evidently of an animal nature. Thefe two 
fubftances he dexteroufly feparated from each other by a 
very eafy chemical analyfis ; by the gentle operation of 
which they were exhibited diftimffly to view, without any 
material alteration from the aftion of the folvent, or in- 
ftrument employed for that purpofe- On an entire (hell, 
or a fragment of one, contained in a glafs veffel, he 
poured a fufiicient quantity of the nitrous acid, conside¬ 
rably diluted either with water or fpirit of wine. After 
the liquor has diffolved all the earthy part of the (hell, 
which may be collefted after precipitation by a fixed or 
volatile alkali, there remains floating in it a foft fub¬ 
ftance, confifting of innumerable membranes of a retiform 
appearance, and difpofed, in different (hells, in a variety 
of pofitions, which conftitutes the animal part of it. This, 
as it has not been affefled by the folvent, retains the 
exaiSI figure of the (hell; and, on being viewed through 
a microfcope, exhibits fatisfaftory proofs of a vafcular 
and organical ftrufture. He (hows that this membranous 
fubftance is an appendix to the body of the animal, or a 
continuation of the tendinous fibres that compofe the li¬ 
gaments by which it is fixed to its (hell; and that this 
laft owes its hardnefs to the earthy particles conveyed 
through the veffels of the animal, which fix themfelves 
into, and incruft, as it were, the me(hes formed by the 
reticular filaments of which this membranous fubftance 
is compofed. In the (hell called porcelain , in particular, 
the delicacy of thefe membranes was fo great, that he was 
obliged to put it into fpirit of wine, to which he had the 
patience to. add a Angle drop of fpirit of nitre day by day, 
for the (pace of two months ; left the air generated, or 
let loofe by the aftion of the acid on the earthy fubftance, 
(hould tear the compages of its fine membranous (frac¬ 
ture, which it certainly would have done, in a more 
liafty orlefs gentle diffolution. The delicate reticulated 
film, left after this operation, had all the tenuity of a (pi- 
tier’s web; and accordingly he does not attempt to deli¬ 
neate its organization. In other fhelis he employed even 
five or fix months in demonftrating the complicated mem¬ 
branous ftrufture of this animal (ubftance- by this kind of 
chemical anatomy. In general, however, the procefs does 
not require much time. 
Of the many Angular configurations and appearances of 
the membranous part of different (hells, which are deferi- 
bed in this memoir, we (hall mention only, as a fpecimen, 
the curious membranous (lru6htre obferved in the laminae 
of mother-of-pearl, and other (hells of the fame kind, af¬ 
ter having been expofed to the operation of the author’s 
folvent. Befides the great variety of fixed or permanent 
colours with which he found the animal filaments of thefe 
ffielis to be adorned, it is known, that the (hell itfelf pre- 
fents to the view a fucceflion of rich and changeable co¬ 
lours, the produffion of which he eafiiy explains front the 
configurations of their membranes. Nature, he obferves, 
always magnificent in her defigns, but Angularly frugal 
in the execution of them, produces thefe brilliant decora¬ 
tions at a very fmall expence. The membranous fubftance 
above-mentioned is plaited and rumpled, as it were, in 
fuch a manner, that its exterior laminae, incrufted with 
their earthy and femi-tranfparent matter, form an infinite 
4 number 
