Mr. Gray on the Structure of the Pkohdes. 407 



ference in the structure of the hinge from the rest of the bivalves, 

 which was first hinted at but not explained by Dr. Leach. 



The valves of most of the species of this family have a portion 

 of the inner part of their shell reflected over their beaks : this 

 portion appears usually to be formed of two plates, which are 

 sometimes very distinct from each other, and have cells between 

 them. 



The valves are connected together by a thin membrane, which 

 often has shelly matter deposited on it in the form of one or more 

 plates. The front part of this membrane appears to be continued 

 along the inner edge of the reflected portion, to its central and 

 most convex part, where it forms a roundish piece of ligamentous 

 matter, which when rubbed otf, leaves a scar that marks its place ; 

 this ligamentous matter appears to perform the office of keeping 

 the valves more steadily attached to each other, and at the same 

 time it affords a very complete fulcrum for the action of the 

 muscles which separate and close the valves. 



To the outer surface of these reflected plates is fixed a muscle, 

 formed of transverse fibres ; this muscle, assisted by the reflected 

 plates, which from their convex shape act as a lever, opens the 

 valves by the contraction of its fibres. It is mostly covered by a 

 membrane arising from the outer edge of the lower, and most 

 extended of the reflected plates, in which membrane one or more 

 shelly pldtes formed similarly to the valves of shells, and called 

 by Linneus accessary valves, are often imbedded; but in those 

 animals that are always enclosed in a shelly tube,* as the Tere~ 

 dines, the muscle is only covered by a thin skin, as the shelly 

 tube protects it. 



Thus the valves of this family are opened in a manner directly 

 the reverse of other bivalves, for here instead of the ligament 

 being external, and the elastic cartilage on the inside of it, the 

 ligament is internal, and the muscle which performs the functions 

 of tlie cartilage is placed on the outer dorsal side of it. 



* Those tubes appear to be a lining deposited on the inside of the hole, f<Jr 

 the purpose of affordinp a smooth surface, for the protection of the very mueli 

 elonf;ated body of the animal ; they are often furnished with incomplete tr«nR> 

 verite diskepii^icntk, especially at the pobterior cxlreniity. 



