Chap. V. CONCH 
Of the Con- ferved on univalve (hells, which run in a.tranfverfe di- 
ftituent region to the fucceflive growth of the convolutions of 
Sh-lls &c fy* re > have been denominated varices * by Linnaeus, 
’ »in allulion to the dilated veins on the bodies of other 
animals. They are compofed of one or more eleva- 
114 tions, ufually arranged in a line parallel to the axis of 
Formation tL e (hell, and fometimes {lightly oblique. They con- 
counted^" ^ *”he ^ ame fubftance as that of tile reft of the (hell* 
f or , but are thicker and always more elevated than the fur- 
face of the convolutions of the fpire on which they are 
placed. To explain the manner in which thefe eleva- 
tions are formed, we may examine the opening of land 
{hells which have arrived at the laft ftage of their 
growth. This period is marked in thefe {hells by a 
kind of margin of about a line in breadth, which is 
fometimes turned outwards, although the reft of the 
{hell turns on a regular, fpiral line. This refle&ed 
margin never appears in land {hells, but when they 
have reached the laft period of their growth, and when 
it is once formed, the animal of fome fpecies ceafes af- 
terwards to continue the convolutions of its fpire. 
Having now arrived at that period of its growth, when 
it is fit to perform the a£l of generation, it protrudes 
itfelf more frequently from its {hell, and each time it 
returns, a vifeid fluid which exudes from its neck, is 
interrupted and depoftted on the external margin of the 
{hell. The bulk which the anterior parts of the body 
have acquired in confequence of the evolution of the 
generative organs which are contained in that part of 
the body, caules it to prefs more ftrongly than former- 
ly on the edges of the opening of the ftiell, every time 
it protrudes itfelf, and gradually forces the particles of 
teftaceous matter which have been recently depoftted, 
to the external furface, and in a direction quite differ- 
ent from that of the former plane of the fpire. A fhort 
time only is requifite for the complete formation of 
this elevation ; but after it has been formed, if the ani- 
mal has the power of continuing the fpire on the for- 
mer plane, the {hell which had arrived at a larger 
ftze will exhibit from time to time, if the fame procefs 
be repeated, longitudinal projecting ribs, convex or 
bent, exaCtly fimilar to the external fwelling of the 
opening of the ftiell, and analogous to the varices which 
are feen on fome fpecies of marine {hells. 
Limited to This power of continuing the fpire, after the forma- 
iea lhells. fft on 0 f the eminence at the opening, is peculiar to fea- 
lliells. No farther increafe, after it is once formed in 
land (hells takes place. The young of fome fea-lhells, 
as fome fpecies of murex , alfo poffefs this faculty of 
continuing the growth of 'the ftiell after the formation 
of fimilar elevations, even from the earlieft period of 
their exiftence, and long before it can be fuppofed that 
the organs of generation are evolved. This no doubt 
depends on fome peculiar ftruCture or organization of 
the animal, and particularly on thofe of the anterior 
1 16 parts of the body. 
Tubercles Tubercles. Many (hells are furniftied with tubercles, 
produced which are produced by the fame organs as the reft of 
^ ame the ftiell. The flefhy protuberances which are placed 
on the external furface of the neck of the animals 
which inhabit them, ferve as a mould, and according 
as there are more or lefs of thefe tubercles, while the 
animal enlarges the turn of the fpire, and increafes its 
ftiell fo much, there is the fame number of protuber- 
ances in the convolution. Thefe protuberances, while 
OLOGY, ATS 
they remain on that part of the body of the amrttal on Of the Con- 
which they Were formed, are hollow, and during the of 
remaining part of its exiftence, as the body enlarges, shells, See. 
they are partly hollow, and partly folid, being filled up — «— y—** 
with teftaceous matter, excreted from the body of the 
animal* and then the internal furface of the {hell be- 
comes fmooth and even» 
Spines , and fringed or irregular protuberances, with 
which fome lhells are armed, have, according to all 
appearance, the fame origin as the other inequalities 
on the external furface of (hells. They are ufually 
formed at the end of the different fucceflive periods of 
the growth of the ftiell. This will be fufficiently ob- 
vious, if we trace the whole feries of wrinkles or ftriae 
which run parallel to the circumference of the open- 
ing. Thofe which arife immediately from the ribs or 
varices, are produced by particular organs which fur- 
round the extremity of the neck, and ftretch out from, 
every part of its circumference, fecreting a teftaceous 
matter, which partly forms a (heath around them, gra- 
dually increafes in thicknefs, and fuccefiively affumes 
the form of that part of the body which in fome mea- 
fure ferves the purpofes of a mould. In all the fpecies 
of murex, which are furniftied with fpines, the eleva- 
tions called varices or ribs* as well as the fpines with 
which they are armed, are placed on the (hell at equal 
diftances 5 and the intermediate parts of the fhell, al- 
though frequently grooved or ftriated, are not furnifti- 
ed with fpines. This uniform obfervation, not only 
in {hells belonging to this genus, but alfo in almoft all 
fpinous lhells, proves, that the fpines, as well as the. 
ribs, are to be confidered as formed by the margin of 
the anterior parts of the body, which is renewed in 
the fame proportion as the change in the pofition of 
this part of the body takes place. It proves alfo* that 
the formation of lhells is entirely owing to the fuccef- 
five and regular enlargement of the animal •* and that 
it increafes every time it is difplaced from the whole tty 
extent in breadth of the anterior part of the body, the Spines pro* 
margin of which only being furniftied with long fleftiy 
proceffes or fringed appendices, is in reality the only ^[L-s P ° 
part which produces them on the {hell at each period 
of its increafe. In the fame way is formed the beak 
or prolongation of the {hell, which terminates the in- 
ferior extremity in the form of a canal. This canal is 
produced in all fhells in which it exifts, by a cylindri- 
cal organ, fufceptible of extenfion and contraction, and 
which, according to fome naturalifts, is employed by, 
the animal as a kind of feeler, and occafionally to at- 
tach itfelf to folid bodies. It excretes and depofits a 
teftaceous layer which ferves it as a kind of (heath, in. 
a fimilar manner to the production of fpines. 
It is eafy to explain the formation of the grooves or Formation 
elevated ribs which are found on the outer furface of of ribs and 
other fhells 5 while the whole of the internal furface is g iooves - 
fmooth and polifhed. In bivalve {hells, which exhibit 
this ftruClure, the whole anterior furface of the animal 
is grooved or channelled in the fame way 5 and from 
this the (hell derives its fliape and ftrU&ure. In thefe 
{hells it may be obferved, that it is only the anterior 
margin that is grooved on the internal furface ; be- 
caufe, in the progrefs of the growth of the animal, 
that part of the body which prefents a fmooth equal 
furface has advanced, and nearly filled the whole of 
the (hell 5 and the teftaceous matter fecreted from this 
3 O 2 part 
