4-7 
G O N G H O L O 'G Y. 
^ftituent ^ er - ia ^ k > rers Js alvi ' a y s Akvious ; for if they are not 
Parts of they exhibit everywhere a uniform colour, and 
Shells, &c. neve i' variegated, like what appears externally. By 
T’*—-' removing with a file any part of the external furface 
of the .(hell, the layers which appear immediately under 
the furface, arc thofe which have been furn hired by the 
body of the animal ; while thofe on the fur 
ilfelf, 
93 . 
Formation 
of ft rise. 
99 
- Growth in- 
tenupteJ. 
I oo 
Evamote 
onf this. 
ufually more variegated than the reft, owe their forma- 
tion to the veffels about the neck, and have been formed 
in the way already defcribed. 
ft he -growth of (hells, being proportioned to that of 
the inhabitant, proceeds aknod imperceptibly. In 
mod (hells, however, it is eafy to diftinguifli the dif- 
ferent additions which they have received ; for they 
are marked on their convex furface with different emi- 
nences which are parallel to each other, fimilar to lines 
of different degrees of depth, which give the (hell a 
fibrous dru&ure. Thefe elevations are called Jlrice, 
may be traced through the whole of the (hell in bi- 
valves, and in the longitudinal direction of thofe which 
have a fpiral form. From the flighted obfervation of 
the manner in which (hells are formed, it is eafy to fee 
that they receive no addition, without leaving, in a 
greater or lefs degree, fome traces of thefe inequalities; 
for every fmall addition of tedaceous matter which is 
made, mud be attached to the old part of the (hell, 
which confequently mud be more elevated than the 
former, whatever be its thicknefs, when the enlargement 
of the animal requires the formation of the latter. 
( bus, the (hell will be marked with a great number of 
ihefe drise, parallel to each other, which may be di- 
dinftly feen on many different fpecies. 
Every (hell has ufually lome of thefe eminences at 
greater didances, and more elevated than the others. 
By thefe the different periods when the (hell ceafed to 
increafe, or rather thofe when its growth was inter- 
rupted, are marked ; and they have fome degree of 
analogy with the different (hoots from the branch of a 
tree. The heat of fummer or the cold of winter inter- 
rupting the growth of the animal, at lead among fuch 
as -are tedaceous, which live on the land, or inhabit 
rivers in temperate regions, the (hell is not enlarged in 
extent during thefe feafons. It is otherwife, however, 
with regard to its thicknefs, for there is continually 
exuded from the body of the animal, fmall quantities 
of fluid, which incjreafe its thicknefs. Hence it is, 
when the (hell begins to increafe in extent, the edge 
to which the new portion is cemented, is much thicker 
than when the growth was gradual and imperceptible, 
and confequently the place at which the growth com- 
mences after a long interruption is didinguidied by a 
more elevated ridge, than in the continued pregreffive 
additions which it receives. The numerous indances 
of this interruption in the growth of (hells, will occur 
to the attentive conchologid in the progrets of his 
refearches. We have at prefent in our poffeflion, a 
fine illudration of the fame thing, in a fpecimen of 
murex ramofus. The animal, it would appear from 
the original .part of the (hell, had been for fome time in 
a fiekly or unhealthy date ; for it has undergone many 
of the changes to which dead diells are fubjedt. It 
has lob its enamel ; it feems to have undergone fome 
degree of decompofition, and fome fpecies of ferpu/a 
.and other parafitical animals had made it their abode; 
but from this fiekly date it Teems to have recovered, 
Chap. V* 
and acquired great vigour ; for the next audition which Of the Con- 
is made to the (hell, is equal to its original bulk. It is fthutent 
clean, entire, and in perfect prefervation, forming a fin- Q 7 >a l ts 
gular contrail with the old (hell. . . tx B ‘ ^' c ‘. 
The place at which (hells begin to increafe, after i 0I 
the growth has been for fome time interrupted, may Diftifiguifli- 
be diftinguidied by a difference of colour in the (tripes ed by tile 
with which the (hell is ufually marked. In thefe 
places, black or brown llripes exhibit more vivid 
colours, and fometimes even little different from thofe 
on the red of the fuperior furface of the diell. The 
caufe of tliis change is not difficult to trace, if we re- 
coiled! that the fecretory organs which prepare the co- 
louring matter, at lead in the helix netnoralis , have 
their origin at fome didance from the extremity of the 
neck, from which we have feen that the firh layer of 
diell which is traced to the extremity itfelf, diould he 
of a different colour from that of the dripes ; but as 
the increafe of the animal oceafions the dripes to be 
formed under the fird (hell, during which it is dill 
very thin, and confequently tranfparent, it does not pre- 
vent the (hell produced under it, of a black colour, to 
appear fo. But when the animal has ceafed to grow 
for fome time, it then increafes the thicknefs of the (hell 
lad formed, fo that the (hell which is next produced 
from the colouring matter, when the animal begins to 
grow, being laid on one part of the old (hell much 
thicker and lefs tranfparent, the colour of thefe dripes 
mud appear lefs bright, and therefore different in thofe 
places, from the other parts of the diell. . z 
In taking a review of what has been faid concerning Colours 
the production of the colours of diells, it mud appear owing to 
that thefe rays or coloured lines are owing to glands^ S lr ‘^ s 
which fecrete the colouring fluid, and which are ar- n ,./]T 
ranged on the anterior edge of the neck, while the 
pofterior part furnifhes only a fluid of a different co- 
lour, and ufually lefs deep than the fird. By means of 
this principle it is not difficult to account for the ar- 
rangement of the different colours which are fo fplen- 
didly exhibited among this clafs of natural objedls. 
Thefe colours may be reduced to one or more, which 
are more vivid on a lighter ground ; to coloured, cir- 
cular bands on a ground of a lefs vivid colour, or pure 
white ; to longitudinal lines, round or fquare fpots, 
and in a regular, or irregular, zig-zag form. All 
thefe may be eafily explained, according to the princi- 
ples which have been laid down, the application of 
which, from what has been faid, will not, we hope, be 
found difficult. 
But from this mode, which is the mod general in Co }* 0 f 
the production of the colours of (hells, there are cer- porcelain 
tain deviations. In that divifion of (hells which is (hells, 
made by fome naturaliits, and which is didinguidied 
by the names of porcelain Jbei/s , on account of the fine 
enamel with which they are covered, there are two 
fets of colours, which are difpofed in a parallel direc- 
tion to each other. The external range of thefe co- 
lours is owing to a peculiarity of ftruclure in the animals 
which inhabit them, different from that of ether teda- 
ceous animals, and to an operation which does not 
take place in other diells. In thefe diells, the colour- 
ing matter feems to be depofited in two different ways, 
and at two different periods. In the fird proeefs, 
when the body of the (hell is formed, the colouring 
matter is excreted from the glands, in the fame way 
as 
