214 THE ISrLUENCE OF INANIMATE SUBEOUNDINGS. 



is, relatively speaking, of considerable thickness, very tenacious, 

 and perfectly impermeable by water ; and there can be no doubt 

 that, merely by its resistance to the action of water, it serves to 

 protect the calcareous inner portion of the shell against its 

 solvent and destructive effects. Indeed, this is proved by the 

 subjoined sketches of one or two shells, of which the calcareous 

 layers have been deeply eaten into, while the organic cuticle 

 hangs about the shell in thin rags, as it were, above the holes 

 or pits (fig. 63). The qiiestion now is: How did erosion first 

 begin in these cases? For, though it would be perfectly 

 intelligible that the face of the calcareous layer, when once 

 laid bare, should be easily eaten into by the action of the 



Fig. 63. — Shells of living molluscs partly eroded, ti, Melania ; 6, Navicella ; c, NeHtina, 



carbonic acid in solution in the water, it must have been per- 

 fectly protected against this action by the cutic'e so long as it 

 remained perfect. 



Microscopic examination of the shells here depicted has 

 shown that the substance is penetrated all over, but more 

 especially in the immediate circumference of the eroded spots, 

 by innumerable perforations caused by a minute boring Fungus; 

 these perforations are usually perpendicular to the surface of 

 the shell, and might easily be supposed to be a normal- pecu- 

 liarity of its texture. These boring fungi are also to be found 

 in shells which appear to be perfectly sound, being still com- 

 pletely enclosed in their brown cuticle, while others are already 

 slightly eroded at the apex. This supplies us with the required 

 explanation. The boring fungi, of which the spores are con- 



