170 SUPPLEMENT 
ling or thickening of the right edge of the aperture in the uni- 
valves, or of the entire free edge in the bivalves, which is to 
be noticed at very different intervals in the extent of the spiral 
cone; while between these intervals the shell is more smooth 
and slender. Are these intermissions themselves determined 
by that of the activity of the digestive organs, or by that of 
the generative organs? This‘is a point not easy to ascertain or 
to decide, but which might nevertheless be plausibly referred 
to the state of either of these organic systems. We may con- 
ceive, in fact, that during the period of generative activity, the 
vital congestion being carried to the organs of generation, 
would proportionally diminish that of the skin, and of the 
cretaceous excretion, and then that the growth of the shell 
would go on as usual, from whence are the interme- 
diate spaces at the swelled or thickened parts; but when 
this congestion had ceased, it would be directed towards 
the skin, whence an accumulation of calcareous matter 
at the edge of the aperture, which would produce simple or 
ramified swellings, according to the simplicity or the sub- 
division of the edges of the productive mantle. The rarity or 
the frequency of these subdivisions would determine the num- 
ber and distance of the swellings, sometimes very close and 
crowded, as in scalaria, lyra, and certain species of venus, or 
widely intervalled, as in tripterous and dipterous murices, and 
triton, where these swellings in the growth of the spire are 
regularly disposed, three in number, one on each side, and one 
medio-dorsal ; or two in number, symmetrical, one on each 
side, which gives to the shell generally considered a flatted 
form; or two in number, not symmetrical. But we must re- 
mark that these swellings are always formed of vitreous and 
not of lamellary substance. 
When the animal has arrived at the term of its growth, and 
within limits of bulk sufficiently variable, its shell is always 
terminated by a swelling or thickening in the species of which 
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