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, 

 and 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 discerned ; 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 



