6 MOLLUSCA. 
added to the inner surface of the skin, thus leaving the in- 
terposed substance between the new formed portion and 
the fractured edge. Similar experiments, repeated on a 
variety of shells, both univalve and bivalve, by different 
naturalists, leave no room to doubt that shells increase in 
size by the juxtaposition of shelly matter from the common 
integuments. 
Each calcareous layer is more or less enveloped in the 
animal matter which we have already stated as being pre- 
sent ; so that the different layers of successive growth may, 
by various processes, be distinctly exhibited. If the shell 
has been exposed for a short time in the fire, the animal 
matter will appear charred, and its black colour, contrasted 
with the white earthy matter, will indicate the arrangement 
of the different strata; in the same manner as the ivory 
and enamel of a tooth can be distinguished, when subjected 
to similar treatment. The same satisfactory results may 
be obtained by a different process. If the shell be steeped 
in weak muriatic acid, the earthy matter will be dissolved, 
anc the flakes of albumen will remain as the frame-work 
. of the edifice. , 
The layers of growth may often be distinguished on the 
surface of the shell, in the form of striae or ridges, more or 
less elevated, but parallel to the margin of the aperture. 
Other inequalities may likewise be observed on the surface, 
at right angles to the layers of growth, such as ridges, 
knobs, and spines. ‘These last derive their origin from the 
inequalities of the skin on which they have been moulded. 
In some univalve shells, the layers of growth parallel to 
the opening cannot be discernéd ; when exposed in the fire, 
there is little darkening of colour; and when dissolved in 
acids, but a feeble trace of animal matter remains. In the 
