CONCHIFERA. 385 



class of Mollusks, viz. the growth and formation of their shells. 

 Infinitely diversified are the forms presented by their testaceous 

 valves, and equally various the colours which not unfrequently adorn 

 their external surfaces. Some exhibit a beauty and delicacy of 

 sculpture of a most exquisite character ; others, covered with large 

 spines, or festoons of calcareous plates, puzzle the beholder to 

 comprehend how the growth of such parts, in the situations which 

 they occupy, can be effected with so much regularity of arrange- 

 ment. The shells themselves are absolutely deprived of vitality, 

 permeated by no vessels, and as incapable of expansion by any 

 internal power as the rocks to which they are not uncommonly 

 attached ; so that the young naturalist is necessarily at a loss to 

 conceive either the mode of their formation, or the origin of all 

 the gaudy tints and external decorations that render them the 

 ornaments of our cabinets. 



The simple apparatus by means of which shells are constructed 

 is the external membranous layer that invests the body of the 

 mollusk, the mantle, as it has been termed ; and, whatever the 

 form of the shell, it owes its origin entirely to this delicate organ. 



In order to simplify as much as possible our description of 

 the process whereby the shell is formed, it will be necessary to 

 consider it under two points of view : first, as relates to the en- 

 largement of the valves in length and breadth ; and secondly, as 

 regards their increase in thickness, very different parts of the 

 mantle being employed in the attainment of these two ends. 



It is the circumference, or thickened margin of the mantle, 

 alone, which provides for the increase of the shell in superficial 

 extent. On examining this part (Jig. 176, hfjig. 177, e), it is found 

 to be of a glandular character, and moreover not unfrequently pro- 

 vided with a delicate and highly sensitive fringe of minute ten- 

 tacula. Considered more attentively, it is seen to contain in its sub- 

 stance patches of different colours, corresponding both in tint and 

 relative position with those that decorate the exterior of the shell. 



When the animal is engaged in increasing the dimensions of 

 its abode, the margin of the mantle is protruded, and firmly ad- 

 herent all round to the circumference of the valve with which it 

 corresponds. Thus circumstanced, it secretes calcareous matter, and 

 deposits it in a soft state upon the extreme edge of the shell, where 

 the secretion hardens and becomes converted into a layer of solid 

 testaceous substance. At intervals this process is repeated, and 

 every newly-formed layer enlarges the diameter of the valve. The 



