29^ POPULAll BRITISH CONCHOLOGY. 



the shell. The phrase used then is ligament external ; in 

 other cases there is scarcely a vestige of outer ligament, but 

 the cartilage is contained in a pit, or series of pits, within 

 the edge of the shell ; and then it is said that the ligament 

 is internal. In some shells there is not only a very strong 

 external ligament, but also, quite separate from it, and 

 placed in an oblique pit within the shell, is the cartilage. 



A good specimen, of internal ligament, or cartilage, is 

 found in the Pecten, which moves about rapidly, by using 

 its valves as fins, alternately opening them widely and 

 shutting them. This is effected by the adductor muscles 

 and the cartilage acting alternately upon the valves. The 

 latter, contained in a triangular pit, and of great thickness, 

 is pressed into a small space by the contraction of the muscle; 

 and then, the pressure being removed, it throws back the 

 valve with a spring. Tor this reason, shells whose animals 

 have died are found with their valves open ; the muscles no 

 longer having power to contract. In Area and some other 

 species the ligament is spread over an outer area, and the 

 cartilage is confined to grooves in the same area under it. 



THE HINGE LAMINA, 



Or thickened inner edge of the shell, on which the teeth 



