100 FORMATION OF SHELLS OF ANIMALS, ETC., 
the internal membrane, on the contrary, as the mussel 
grows, receives an addition of new membrane to the whole 
of its inner surface. This membrane is perfectly homo- 
geneous, having no markings whatever, excepting close to 
the edge of the valve, where it is continuous with the 
external horny membrane. Nearly the same arrangement 
exists in the lamine of the shell of the oyster, which being 
much less firmly connected together, are more favorably 
circumstanced for the examination of the process of calci- 
fication. The physical conditions favorable to the process 
of coalescence of the minute particles of carbonate of lime 
into a globular form are less interfered with by so close a 
proximity of its membranous and calcified layers as exists 
in the shell of the mussel. Hence I shall confine my 
descriptions almost entirely to the appearances showing 
the nature of this process, as they are found in the shell of 
the oyster. I may observe that I have found those oysters 
which are of the largest size, and which have large cavities 
between their lamine, as represented in fig. 6 a, to be best 
adapted for displaying the development of shell-tissue, and 
that the most successful examination I have made of this 
tissue in its earliest state of calcification have been in the 
month of June. I name this circumstance since the season 
of the year may have a great deal to do with the develop- 
ment of the shells of these molluscs. The common way 
of displaying the structure of these shells is by making 
vertical sections of various parts of them, and grinding 
them sufficiently thin to allow of being viewed by trans- 
mitted hght. ‘These sections are very useful, and indis- 
pensably necessary to show certain facts connected with 
shell-structures ; but they afford no information relative 
to their development. This can only be learned from the 
examination of extremely thin, partially calcified lamine, 
