MOLLUSOA ACEPhJALA. 197 



presents the section of an oyster, shows the situa- 

 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 adductor muscle 

 might, it is evident, be called into play to coun- 

 teract that action ; but this would require a con- 

 stant muscular exertion and a great expenditure, 

 therefore, of vital force. Nature has also shown a 

 solicitude to economize muscular power, whenever 

 a substitute can 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 lig amentum nuc/ice, 

 and being placed on the side of the muscle next to 

 the hinge, allows the valves to separate to the pro- 

 per distance only.* When the animal dies, the 

 muscular force ceases ; but the ligament, with 

 which the muscle is associated, retaining its elas- 

 ticity, allows the shell to open, but only to a certain 

 extent ; and accordingly, this is the state in which 

 we tind bivalve shells that are cast upon the shore, 

 after the soft flesh of the animal has decayed and 

 been washed out, provided the cartilage and the 

 ligament of the hinge are still preserved. 



The P kolas is an exception to this rule ; for 

 instead of its valves being united, as usual, by an 



* 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. 



