474 
Of the Con 
ftituent 
Harts of 
Shells, &.c 
CONCHOLOGY. 
top 
Lower 
valve co- 
leu. efs, 
have hitherto eluded the obfervation of naturalifls, are 
uniform and conftant, as long as the caufes which 
operate in their produftion, continue to aft. At firft 
fight it might be fuppofed that the difference of tem- 
perature is the caufe of the difference in the intenfity 
of colour, in fhelis produced in different climates. It 
might be fuppofed too, that the different depths at which 
fhelis are found in the ocean, the medium in which they 
live being thus very different, would occafion great di- 
verfity in the colour. Near the furface, where the 
heat is greateff, if the operation of this caufe were con- 
fiderable, the colours of Ihells fhould be expefted to be 
moft vivid, and as the depth increafed, at leaff to a cer- 
tain extent, the intenfity of colour fhould be diminifh- 
ed. But it has been obferved in bivalve fhelis which 
are found at great depths, fuch as feme fpecies of oy fi- 
ler and fpondylus, that the lower valve which is at- 
tached to the rock, is almcjl always white or colourlefs, 
while the upper valve often exhibits bright and vivid 
colours ; but this difference cannot be aferibed to the 
difference of temperature, for in both valves it mull be 
the fame', the matter fecreted for their formation is 
prepared by the fame organs, and is depofited in a fi- 
milar manner ; and indeed they are altogether placed 
in the fame circumffances, and have been expofed in 
their produftion and growth to the operation of the 
fame caufes, excepting that the upper valve is expofed 
to the rays of light, and is therefore coloured, while the 
lower valve is removed from the aftion of this caufe, 
and is colourlefs. 
The fame difference is obferved in the valves of 
other fhelis, which are produced in fimilar circumffances. 
The different fpecies of pholas which make their abode 
in calcareous or coral rocks, and the teredo nova /is, or 
fhip-worm, which pierces wood, and makes it its habi- 
tation, are ufually colourlefs. Thofe teflaceous ani- 
mals too, which live at great depths in the ocean, and 
are thus far removed from the influence of light, are al- 
fo diffinguifhed by very faint colours, or are entirely 
white. 
Sect. IV. Of the Formation of the Umbilicus , Pro- 
tuberances, &c. 
We have hitherto confidered only the general for- 
mation of fhelis. In the prefent feftion we fliall treat 
of feme other circumffances which produce variations 
jn their external figure. Such, for inftance, is the for- 
mation of the umbilicus, of fpin'es, tubercles, ribs, and 
X1I other protuberances. 
Four claffes Umbilicus. Univalve fhelis, which are furnifhea with 
cf lpiral a regular fpire, may be divided with regard to their 
form, into four claffes; namely, fhelis having a dife, cy- 
lindrical fhelis, turbinated, and ovoid or egg-fhaped 
fhelis. Thefe four forms are the moft common which 
fpiral univalve fhelis affume, and they depend on the 
manner in which the turns of the fpire are applied to 
the common axis, and the difference of their arrange- 
ment. They derive their primitive figure from the 
fmall fliell while it is yet included in the egg, and pro- 
bably from that of the external organs of the animal 
which is contained in it. But although all univalve 
fhelis may be referred to one or other of thefe four prin- 
cipal forms, they exhibit a great variety of flighter 
(hades of difference. Let us now fee in what way 
no 
and fhelis 
included in 
other bo- 
dies. 
ftieds 
it may be conceived that the bodies of the animals 
which inhabit univalve fhelis, give them a fpiral form. 
If eve can fuppofe that from the firft produftion of 
thefe animals, when they begin to be developed, the 
fibres of one part of the body, futh as thofe of the ex- 
ternal furface, are longer than thofe of the oppofile 
furface, it is obvious that the body of the animal con- 
tinuing to increafe, according to this original tendency, 
will affume a curved form, the concave part of which 
will be on that fide where the fibres are fhorteft ; and 
if the long fibres on the external furface, and the fhort 
fibres on the internal furface, continue to increafe in the 
fame proportion, this muff give the body a fpiral form; 
but in this cafe, the different convolutions of which the 
animal is compofed, will be in the fame plane, and can 
only apply to a fmall number of (hells included in the 
firit divillon, namely thofe which are characterized w ith 
having a dife. 
The convolutions of the fpire which are deferibed by 
the Ihtil of univalve teflaceous animals, and the body 
which ferves as a mould for thefe, are difpoied in dif- 
ferent planes. Some other caufe, therefore, muff ope- 
rate in producing this deviation. Between the two fur- 
faces of the body of the animal, which is fuppofed to 
be furnilhed with fibres of different lengths, it is eafy 
to conceive two other furfaces direftly oppolite to each 
other, an upper and an under furface, each of which is 
included between the two preceding furfaces, but of 
fmaller extent ; and it is eafy to conceive farther, 
that thefe two latter furfaces are fo formed, that the 
fibres of the one are longer than the correlponding and 
oppofite fibres of the other. According to this Itruc- 
ture, the body of the animal will tend to that furface 
on which the fibres are fhorteft, and thus deferibe, du- 
ring its developement, a fpiral line in different planes, 
in proportion to the difference of tenfion between the 
fuperior and inferior furface of the body, as well as be- 
tween the lateral furfaces. 
The form of the fliell depending on the external form 
of the body of the animal, the umbilicus, which is a 
different cavity from that of the opening of the (hell in' 
which the animal is contained, and which is feen on 
the inferior furface of fome Ihells, in the centre of the 
convolutions of the fpire, depends entirely on the plane 
on which the animal has formed the additions to its 
(hell. If the plane of thefe convolutions has been di- 
refted round a conical or elliptical axis, and each con- 
volution of the fpire be more or lefs diftant towards the* 
centre of the fhell from this hollow point, a fliell may 
be thus formed, whofe umbilicus w ill be more or lefs 
open, according to the greater or lefs degree of fepara- 
tion which the animal mud give to the convolutions of 
the fpire, correfponding to its ftrufture. An oppofite 
effeft w ill be obferved, if the increafe of the convolu- 
tions of the fpire is fuppofed to take place round an 
axis which is fo frnall as to permit them to come in 
contaft with each other. In this cafe no cavity will 
be formed in the centre, r.o appearance of umbilicus 
will be feen. But if we conceive that the animal, in 
enlarging ilfelf, turns round a folid of a curved figure, 
in place of the conic axis above alluded to, and that 
the end of this folid is at the fumrcit of the (hell, it is' 
obvious that an opening or an umbilicus of the fliape 
of this folid, will be formed in the (hell. 
Ribs, The longitudinal elevations which are ob- 
ferved 
Chap. V. 
Of the Con. 1 
ftituent 
Parts of 
Shells, &c. 
111 
Owe their 
(hape to 
the form 
©f the ani- 
mal. 
n 3 
Umbilicus 
produced. 
