Chap. V. CONCH 
Of the Con- as in other teflaceous animals; and it is arranged ac- 
ftituent cor ding to the difpofition of the glands on the body of 
Parts of t jj e an i ma i s . At this period of the procefs, the (hell is 
Shells ^, ^ a mo derate thicknefs, and much lets than what 
it afterwards acquires, when completely formed. On 
the external furface of the (hell firft formed, another 
layer is depolited, which is more compact than the 
firlt, in fome places thicker, and ulually variegated 
with different colours. The external furface of the (hell 
being thus completely covered with this fecond layer, 
the original colours are concealed ; and if the lame 
lhell were examined at different periods of its forma- 
tion, it would appear like two diitinCt fpecies. The 
Aneaer- organs which are employed by the animal in the pro- 
nal layer duffion of this fecond layer of lhell, and fet of colours, 
farmed. are two foft, membranaceous wings, w'hich being pro- 
truded from the opening of the lhell, completely cover 
the whole of its external, convex furface. Thefe two 
wings, which are quite diftindt from the glandular 
ftrudture above the neck of the animal, which is litu- 
ated a little lower, are alfo provided with glands, 
which furniih colouring matter, ufually different from 
that which is furnillied by the glands of the neck ; and 
it is the upper furface of the wings, which is alone 
provided with this glandular ftruCture. This furface, 
when this part of the animal is protruded from the 
-Shell, and extended over it, comes in contact with the 
external furface of the latter. Hence it is, that thofe 
membranaceous organs depolit on the firft formed and 
coloured layers of the lhell, new layers of teflaceous 
matter, which is differently coloured, and diverfified 
with entire fpots, either circular, or in a waved direc- 
tion, which are fometimes of a more yivid tint than 
that of the ground, or white upon a dark ground, or 
brown upon a yellow ground ; or are compofed of 
Straight lines, or curved, or interlaced with each other, 
reddilh, brown, yellow or white, on different coloured 
grounds, or in dots or points whofe lhades and arrange- 
ment are not lefs diverfified. 
This mode of the formation of the external layer of 
J ‘ porcelain (hells, has been proved by the aCtual obferva- 
tion of fome naturalifts. In fome fpecies, a longitudi- 
nal line of a paler colour is obferved on the convex fur- 
face of the lhell. This is afcribed to the junction of 
the two wings of the animal, where a fmaller quantity 
of colouring matter has been depolited, or where the 
lhell has been lefs completely covered with the protrud- 
ed part of the animal. But the exiftence of this fecond 
layer is ftill more diftinCtly proved by mechanical 
means. The external layer may be removed by means 
of a file, and the lhell reftored to its original ftate ; and 
then the colours which it firft received are brought in- 
to view. This circumftance is ftill farther demonftra- 
ted by an attentive examination of different fpecies of 
(hells, and particularly the cypraa argus. In examin- 
ing this lhell, there are obferved under the external 
layer, which is of a yellow' colour, fome flight traces of 
four tranfverfe bands of a brown colour, which furround 
the lhell, and which muft have been formed previous to 
the more fuperficial yellow layer. By a more minute 
examination, it will appear that the circular fpots with 
which the external yellow layer is marked, have been 
pofteriorly formed to this layer ; and finally, on the four 
turns of the fpire forming a flight projection at the bafe 
of the lhell, there are fome brown, circular fpots, which 
Voi., VI. Part II. 
O L O G Y. 473 
are quite fuperficial, and which fometimes include two Of the Con- 
turns of the fpire, which could not happen if the yel- p.^uof 
low colour had not been prior in its formation to thefe ^ Cm 
circular fpots. If the colouring matter of which thefe y— =# 
fpots are compofed, had been depolited at the time that 
the different parts of the fpire were formed, one fpot 
could not have included two turns of the fpire at the 
fame time. 10 g 
This effect of communicating a new fet of colours Shell be- 
to tire external furface of the lhell, is not the only one toraes 
which is produced by the membranaceous ftruCture of^ 1 ^^ 
the animal which inhabits the porcelain and other (hells. cau f e . 
The form of the lhell is alfo changed in a remarkable 
manner, a great quantity of teflaceous matter being de- 
poiited on the l’urface of the opening, which then af- 
fumes a confiderable thicknefs. The turns oi the fpire 
are incrufted, and fometimes difappear on the out fide 
of the (hells ; and wrinkles, furrows, and even tuber- 
cles, which exift on the furface of fome fpecies, are alfo 
formed. The furface of cypraa pediculus exhibits circu- 
lar Arise which did not originally exift, and which owe 
their formation to this caufe. In other fpecies, the 
furface is marked with projecting points or tubercles, 
which are produced in the fame manner as the circular 
Arise of the former, and which alfo depend on the ftruc- 
ture of the membranaceous wings of the animal, and 
the teflaceous fubftance which is fecreted and depolited 
from their furface. Thus, it appears that porcelain 
Ihells, and thofe of fome other fpecies, are formed at 
two diftinCt periods. It is during the fecond period 
of the procefs that the colour of the complete lhell is 
formed. In farther illuftration of this point, of the for- 
mation of Ihells of this defeription at two different pe- 
riods, one or two examples may be given of the differ- 
ence which takes place, when the laft layer formed is 
removed. In the cypraa exanthema , the lhell is ferru- External 
ginous, with whitilh round fpots and eyes ; but when layer re- 
the outer coat is worn off, it becomes barred or teffe- move< b 
lated with brown or blue. The cypraa arabica , as its 
name imports, exhibits characters on its furface, having pe^ 5 ^ 
fome refemblance to Arabic letters. The ground on 
which thefe characters, which are of a browm colour, are 
placed, is whitilh or bluilh ; but when the outer coat 
is worn down, the (hell is fometimes bluilh with brown 
bands, or pale with darker angular fpots and lines, 
brown mixed with violet, or reddilh blue. Io8 
But befides the caufes which have been mentioned Effefts of 
concerning the production and variety of the colours of light on 
Ihells, arifing from the difference of ftruCture in the or- 
gans which fecrete the colouring matter, and the chan- 
ges to which thefe organs are fubjeCted in the growth 
of the animal, the effeCts of light and heat, altogether 
independent of the animal itfelf, are probably very con- 
fiderable. Trvo individuals of the fame fpecies, the 
one from the Mediterranean or European feas, and the 
other from the tropical regions, exhibit very different 
lhades of colour. The colours of the inhabitant of the 
torrid zone are always more bright and vivid than 
thofe of the native of more temperate climates. The 
two fhells, although fimilar in form, fize, and other 
characters, are uniformly different in the intenfity of 
their colours. Thefe differences, which have led con- 
chologifts to increafe the number of fpecies, obvioully 
depend on the aCtion of the climate, and particularly 
of light, on nourilhment, and other circumftances which 
3 O have 
