STRUCTURE OF SHELLS. 



213 



their growth ; and it is plain that if freely creeping or swimming 

 creatures are exposed to it they will probably in most cases be 

 destroyed. Sometimes, however, these are capable of escaping 

 its effects by their own strength. 



This occurs, for instance, in the case of many Mollnsca, of 

 which the shells ai-e often much injured by erosion. All 

 conchologists are well aware that the shells of fresh-water 

 Mollusca in particular are generally more or less eroded; in 

 many places it is very difficult to find any number of certain 

 species of shells that are not thus worn away, and the process 

 commonly begins at the apex of the shell. It can hardly be 

 doubted that there are very various causes for this ; in many 

 cases, which I myself have observed and studied minutely, 



c 



FIG. 62. Oblique section through the shell of a fresh-water mussel, Unio. a, Cuticle, ex- 

 ternal and organic ; 6, the Prismatic Layer n:xt to it ; c, XacreouB Layer, slightly 

 magnified. 



two causes have combined namely, the boring powers of the 

 filaments of certain Fungi and the constant wear of fresh- 

 water currents, by which, in the Philippines, the shells of 

 various species of Melct-nict, ^favicellft,and ^eritina are attacked. 

 How this occurs can only be understood by briefly studying 

 the structure of auch shells. 



In the shells of Bivalves as well as in those of Univalves 

 three typical layers of structure may be distinguished : the outer 

 one consists invariably of purely organic matter, known to 

 conchologLsts as the epidermis ; the two inner layers consist 

 of calcic carbonate, combined with a very small quantity of 

 feebly developed organic matter. The outer layer of these two 

 is commonly designated as the prismatic layer ; the inner one 

 as the mother-of-pearl or nacreous layer. 101 In most of the 

 Mollusca living in fresh water the external organic layer 



