110 PORCELLANEOUS SHELLS. 



pends altogether on the extent, particular form, and 

 position of the secreting organ. On its exclusion from 

 the egg, the animal has already a small portion of shell 

 formed. The simplest case is that in which this rudi- 

 ment of a shell is a concave disc. We may conceive 

 the animal, covered by its mantle, to expand the border 

 of this organ, and extend it beyond the edge of the 

 shell, where it then forms a new layer of shell ; and this 

 new layer, being applied to the inner or concave surface 

 of the original shell, will, of course, extend a little way 

 beyond its circumference. The same happens with the 

 succeeding layers, each of which, being larger than the 

 one that preceded it, projects in a circle beyond it ; and 

 the whole series of these conical layers, of increasing 

 diameters, forms a compound cone, of which the outer 

 surface exhibits trans versed lines, showing the succes- 

 sive additions made to the shell in the progress of its 

 increase. 



Of this form of structure, the patella, or limpet, is 

 an example. This animal lives on the shores of the sea, 

 and constantly on those parts which are alternately 

 covered and left dry by the waters. None appear to be 

 known belonging to the fresh water, nor have any been 

 observed even in the mouths of large rivers. They are 

 almost constantly fixed on rocks, and sometimes in ex- 



