408 Mr. Gray on the Structure of the Pholades. 



From the natural station and habits of these animals, which are 

 usually confined to holes in rocks or wood of nearly their own 

 size, the valves of their shell are only endowed with a very 

 limited degree of motion, especially in some species, as in the 

 P kolas pusiUuSy where the basal margins are connected together 

 by a thin membrane, which has a shelly plate imbedded in it, 

 similar to that of the dorsal membrane. 



In most other Conchophorous Mollusca the valves are closed 

 by two adductor muscles, the posterior of which is mostly at- 

 tached to the posterior surface of the shell. I have however, ob- 

 served it in a very beautiful specimen of Pholas costata, attached 

 to a plate-like process, similar to those to which the muscle is 

 attached in the genus Cuciillcea ; and in the Pholas Dadylus^ this 

 muscle is placed obliquely, so that the scar left by its attachment 

 in the left valve is quite close to the dorsal margin, while that in 

 the right valve is placed at some distance from it ; the anterior 

 adductor muscle is placed just before the hinge, or the anterior 

 dorsal margin of the shell, at the hinder part of the gape. These 

 two muscles appear also to be assisted, as in some other bivalves, 

 by a bundle of muscular fibres, which is situated at the posterior 

 basal edge of the submarginal scar, just at the angle where the 

 syphonal scar commences, the latter sometimes leaves a con- 

 siderable muscular impression. 



From the inner part of each of the valves of this genus, just 

 under the nucleus, arises a process, which from its being usually 

 somewhat sickle-shape, maybe called i\ie falciform process. It 

 has usually been called by all Conchologists a tooth, but it is not 

 placed similarly to the teeth of other bivalves. I am inclined to 

 difier from several celebrated Conchologists, who consider that 

 the hinge of this genus is destitute of teeth, as there are always 

 one or more ribs on the hinge margin, which answer the same 

 purpose as teeth in other shells, keeping the valves in their relative 

 position one with another; I am therefore inclined to call (hem 

 teeth. 



These processes vary slightly in their shape in the different 

 species, in Pholas costata and P. Dactylus^ &c. they are ex- 

 panded at the end and exhibit several concentric striae. On Pholas 



