216 



POPULAR CONCHOLOGY. 



becomes an entire mass of flint. Both valves are attached 

 to the animal inhabiting them on their inner surface, merely 

 by the insertion of muscles, and all along their outer edge 

 by a skin, which is organically connected with the border of 

 the mantle. This skin or epidermis covers the outer 

 surface of the valves, and sometimes terminates in hairs or 

 silky filaments, for example, amongst the conchifera, in the 

 genus Area and Pectunculus : its texture is horny, and in 

 the My a and Lutraria it encloses the siphons also. 



The valves are fastened together by means of an indian- 

 rubber like band or ligament, which, by its elasticity, both 

 holds them asunder, and acts as an antagonistic force to the 

 muscles which close them, and this whether it be situated 

 on the outside or inside the valves. Where the ligament 

 connects these two valves, these latter often possess besides, 

 tooth-like processes, which lock into each other ; hence this 

 connection of the valves by means of the ligament and teeth 

 is called the hinge. The valves are closed by means of one 

 or two specific muscles, the impression of which can be 

 readily seen on the inside of the shell. 



As an organ of locomotion, the conchifera use the so- 

 called foot, an extremely muscular appendage issuing from 

 the animal in an oblique direction, and having the power 

 of extension. It is of various forms ; sometimes com- 

 pressed at the sides, and resembling a tongue, or it is like 

 a hook, or hatchet, or worm ; and its adaptations are as 

 various as its form, serving the animal for creeping, spring- 

 ing, and boring into sand. In many stationary species the 

 foot is rudimentary or wanting ; in others there is found 

 at its base a glandular organ, which forms separate horny 

 filaments, called the beard or byssus, by means of which the 

 animal attaches itself to wood, stones, or the shells of other 

 mollusca. In these the foot is usually attenuated an 



