*o MANUAL OF THE MOLLUSCA. 



Muscles of a third kind are attached to the shell. The valves 

 of the oyster (and other mono-myaries) are connected by a single 

 muscle ; those of the cytherea (and other di-myaries\ by two ; 

 the contraction of which brings the valves together. They are 

 hence named adductors ; and the part of the shell to which they 

 are attached is always indicated by scars. (Tig. 14, a. a'). 



A 



i place of 



Pig. 14. Left valve of Cytherea chione* 



The border of the mantle is also muscular, and the pL 

 its attachment is marked in the shell by a line called the pallial 

 impression (p ) ; the presence of a bay, or sinus ( s ), in this line, 

 shews that the animal had retractile siphons ; the foot of the 

 animal is withdrawn by retractor muscles also attached to the 

 shell, and leaving small scars near those of the adductors 

 (Fig. 30*). 



The gasteropods withdraw into their shells when alarmed, 

 by a shell-muscle, which passes into the foot, or is attached to 

 the operculum ; its impression is horse- shoe-shaped in the lim- 

 pet, as also in navicella, concholepas, and the nautilus ; it be- 



* Fig. 14. Cytherea chione, L., coast of Devon, (original) ; h, the hinge 

 ligament ; u, the umbo ; I, the lunule ; c, cardinal tooth ; 1 1', lateral teeth ; 

 a, anterior adductor ; ', posterior adductor ; p, pallial impression ; s, sinus, 

 occupied by retractor of the siphons. 



