402 PHILOSOPHY OF ZOOLOGY. 



the fractured edge. Similar experiments, repeated on a va- 

 riety of shells, both univalve and bivalve, by different natu- 

 ralists, leave no room to doubt that shells increase in size 

 by the juxtaposition of shelly matter from the common inte- 

 guments. 



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 mat- 

 ter 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 mu- 

 riatic 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 fire, these shells crack in various directions, but exhibit 

 no trace of a scaly structure. By careful management 

 with the file, the shell may be separated into a central 

 layer contiguous to the skin, and a peripheral layer, both 

 similar in structure, though frequently differing in colour. 

 The shells exhibiting such characters have been termed 



