which prevent, by their want of elasticity, a separation of the valves 

 beyond a given distance, so as to take off from the muscular power 

 anything in the shape of continued strain from elongation. 



So far the observations have reference to vital endowments of the 

 animals inhabiting bivalve shells, but it is chiefly to the character of 

 the mechanical and physical condition of the hinge and ligament that 

 this paper has particular reference. 



If we examine the ordinary species of the Conchifera, we shall find 

 that the majority present certain eminences and depressions which 

 correspond in the two valves, and constitute what is denominated the 

 hinge. This hinge varies materially in different genera : in the genus 

 Spondylus the processes are so perfectly connected, that notwithstand- 

 ing there is freedom of motion for a certain distance, the valves can- 

 not be separated without a fracture of some of the processes ; whilst 

 in Pinna no processes exist, a ligamentous material forming the 

 hinge. 



Generally speaking, however, there are to be found certain eminen- 

 ces and depressions which are more or less developed, which form a 

 kind of joint or hinge, whose presence prevents the valves only from 

 slipping either forward or backward ; the ligamentous structures keep- 

 ing them in contact and completing a true joint. 



It has been already stated that the opening of the shell is a mecha- 

 nical action, effected by the ligament on the valves, on account of its 

 being situated in such a position that its power may be effectively 

 employed. 



It would appear from Dr. Roget's ' Bridgewater Treatise,' vol. i. 

 page 218, that this ligament had one universal position, and that its 

 compression by the adductor muscle was the cause of the elastic 

 power by which the valves are opened, it being there stated that " the 

 closing of the valves produces in all cases a compression of the carti- 

 lage, the elasticity of which tends, therefore, to separate the valves 

 from each other, that is, to open the shell." 



If the most common bivalve shells be examined, such as the cockle, 

 mussel and scallop, it will be seen at once that the former has the 

 ligament external to the hinge, and the two latter have the ligament 

 internal to the hinge, and in all these examples it is a power residing 

 in the ligament that opens the valves. 



Now it is very evident that a power before the fulcrum and a 

 power behind the fulcrum, producing the same effect, must operate in 

 a manner the converse of each other, that is, the ligament before the 

 hinge, in order to open the valves, can only do so by expansion after 



B 2 



