ON THE MICROSCOPIC STRUCTURE OP SHELLS. 9^ 
U thickened l)v successive formations of prismatic structure. The epidermis 
seems to be here present between the successive laminm, as is better seen in 
fig. 10 , which shows the appearances presented by a section of the lip on a 
larger scale. The position of the letter c shows the corrcspondinii parts in 
figs. S aod 10 . r « r 
W. If we revert to the mode in which the well-known shell of Oslrca is 
enlaiged, we shall perceive that there is such an essential conformity be- 
iweea the two cases, as mayjustify us iu founding our general description of 
the prores upon the indications which they present. A cursory inspection 
01 the shell of the Oyster shows us that it Is composed of two different sub¬ 
stances; the principal part of its thickness being made up of the white inner 
tajCT (which I have termed sub-nacreous, § 29), the successive laminsB of 
which ha\^ comparatively little adhesion to each other; whilst the margin of 
each of these laminae is covered on its exterior with a border of a more 
waaPe oroTTOiBh-ycIlow substance, which the microscope shows to be com- 
pwed of prUniRtic cellular structure. Now these cxtcnml layers are quite 
u'scounected, imtead of being continued by tiie junction of their edges one 
with another as in ^nio ; and we thus see plainly that, as long a.-* the shell 
continues to grow, it is only where the new inner layer projects beyond the 
^gin of the old, that the outer layer is generated. Hut we may observe at 
. ® a full-grown oyster exactly that which has just been described 
i« lip of tnio, namely, a succession of lamime of prismatic cellular 
♦L- one upon another, although (like the other layers of 
this sheU) with little on no mutual adhesion. 
shr-ll- tile true history of the enlargement of the 
wuryiVe only of the mantle 
• e power of giving origin to tlic outer layer of tlio shell, whihd its w/io/e 
the inner. Every new production of shell consists of 
old ‘““um of the latter substance, whicli lines the whole interior of the 
So lo or margin of the former, whicli thickens its edge, 
laver n continues to increase in dimensions, each new interior 
Dosed nf I f®*" beyond the preceding, that the new border com- 
to dial !h ^ ** simply juined-on to the margin of the former one; 
other \\ **uocessive formations of the outer layer scarcely uuderlle each 
animal has arived at its full growth, tbo new laminiE 
hand nf ^ coniiiosed of a maiginal 
the iDn i substance, attaidied to the edge of an entire lamina of 
in Otiy^' * hands must now underlie oachother, being ciibcr quite free as 
other as in Unio and most other Bivalves. 
Rallv m that the additions to the shells of Gasteropoda are 
tbatthfc'' ® tipoD tile same plan, although it has been commonly supposed 
linuMl I attached to the edge of the old shell, instead of being con- 
«"««! ovenuMDteraaUurface. 
in the 
those in 
II. Continuation of former Report. 
ihrmep^R families of Bivalve Mollusca considered i 
*hich t) which the lobes of the imuitle are unadberent, to those m 
'0 the it P^^'ally or completely united, we encounter a gradiiu! change 
'urcff /i!vT ®o«ti*bon of the shell. We have seen that in the Margari- 
5 561 fi probably be regarded as the types of the former division, 
»aw rrnf- of the shell is composed of a substance which al- 
t« 4 rim, ‘■haracter; the partitions between the cells being firm, 
altogethef bear a larg .. . 
18*7 enclose. In the group we ai 
_proportion to the cal- 
group we are now to consider, how- 
