MOLLUSCA AC EPHALA . 219 



tion of the hinge L, the adductor muscle A, and 

 the transverse direction of its fibres, with respect 

 to the valves. When these muscles are not in 

 action, the elasticity of the cartilage attached to 

 the hinge is sufficient to separate the valves ; 

 but as they were not intended to open beyond a 

 certain extent, it was necessary to provide some 

 limitation to the action of the cartilage. The ad- 

 ductor muscle might, it is evident, be called into 

 play~to counteract that action ; but this would re- 

 quire a constant muscular exertion, and a great 

 expenditure, therefore, of vital force. Nature 

 has always shown a solicitude to economize mus- 

 cular power, whenever a substitute could be had, 

 and such a substitute she has here provided, by 

 uniting with the muscle an elastic ligament, of a 

 peculiar construction. It has a texture similar to 

 that of the ligamentum nuchce, and being placed 

 on the side of the muscle next to the hinge, 

 allows the valves to separate to the proper dis- 

 tance only.* When the animal dies, the mus- 

 cular force ceases, but the ligament, with which 

 the muscle is associated, retaining its elasticity, 

 allows the shell to open, but only to a certain 

 extent ; and accordingly, this is the state in 

 which we find bivalve shells that are cast upon 

 the shore, after the soft flesh of the animal has 



* This remarkable structure was first described by Dr. Leach, 

 in a paper read before the Royal Academy of Paris. Bulletin 

 des Sciences, 1818, p. 14. See also Gray, in Zoological Journal, 

 I. 219. 



