98 
repqut— 1847» 
ever, the animal basis is usually very small in amount, and cannot be distin¬ 
guished from the calcareous constituent by the simple examination of a trans¬ 
parent section of the shell-substance. When the shell is submitted to the ac¬ 
tion of dilute acid, so as to dissolve away its mineral portion, the only residne 
is usually a set of lamin®, more or loss numerous, of a pellucid membrane of 
extreme tenuity, in which no distinct cc-llnlar structure can be discern^ 
A few detached cells are occasionally to be observed, scattered amongst ib 
folds; but these are destitute of the finn walls w'hich hound the calcarewt 
prisms of the Margaritaccee, There arc certain genera, however, in which 
the structure of the shell is nearly the same as that wbicli characterize the 
last-named family; of these the most remarkublc are Pandora Svlcmyn, 
to which a more particular reference will be made hereafter. 
67- It is not to be inferred, however, from the comparative dcSciency ijf 
animal matter, and from the general nhscnce of distinct cellular structure, in 
the shells of this division of the LaiiiellibraricUiate MoUu^-ca, that they are 
originally formed upon a jjlan altogether ilifferent from that which is followed 
in the preceding groups. For I think that there is good evidence, that, how¬ 
ever dense and apparently homogeneous the texture of the fully-formed shell 
may be, it is generatc<l in the first instance in the same manner as the di- 
stiiictly-cellulav shells; that is, by the agency of the epithelimu of the mant^ 
the cells of which weem to have the power of consolidating themseWfsby 
drawing calcareous matter into their interior. In nearly every family I 
been able, on cnrefnl scrutiny, to discover indications of a cellular structure 
in some portion of the shell. Sometinirs they are extremely distinct in 
nearly the entire sludl, —as in several genem of the family when 
the amount of membrane left after tlic decalcification of the shell by dilute 
acid is very small, and no traces of cells can be discovered in it. In 
cases the distinct cellular structure is confineHl to one jiortion of the shell! 
but instead of being separated from the remainder by an abrupt line ol de* 
marcation, it passes so gradually into it,—the Ijoundarics of the cells beccr 
ming progressively less ami less distinct until tJiey arc altogether lost,—that 
tlie identity in original character between the distinctly cellular structure 
and the apparently homogeneous substance can scarcely he doubted, m 
this we shall pec characteristic examples in Mga, Solemya, and some othtf 
genera. It would seem as if, in the final process of consolidation, tire ^1* 
walls had a tendency to liquefy or dissolve away, unless supported by adoi- 
tional deposits of animal mattiT, and thus to allow of the complete 
their contents. This loss of the original boundaries of the colls, and the con¬ 
sequent obscuration of the real nature of the structui-c, is by no mean? pef“* 
liar to shell. The r**searclic» of I’rof. Owen upon tlic development of the 
teeth, show that it takes place also during the formation of dentiuc! *1''' 
cases in which vestiges of the original cells are preserved being few in Pro¬ 
portion to those in which they are obliterated, but serving indubitably to 
mark the real iifUnrc of the transformation of the soft pulp Into the diwe 
ivory*. And a similar cJinnge seems to take place in the formation of the 
calcareous integument of the Crustacea, which, as we shall hereafter see, 
corresponds with doutino in its principad charactcra. This feature in the 
structure of the shells I ntn about to describe, is therefore one of consl*!^^' 
able interest in General Physiology; and I kIiuII dwell upon it with corre¬ 
sponding minuteness of detail. 
68 . I am strongly inclined believe that the peculiar tenacity of 
calcifictl cellular .subsUuicc in the iV«r^arj‘/<jceri? and certain other genera- '* 
due, not so much to the strength of the original cell-walls, but to the iurer- 
• ‘Odontography,’ IntrcnluctioD, p. xl«. 
