36 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 cytlierea (and other di-myai'ies), 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 ahvays indicated by scars. (Fig. 14, a. a'). 



h 



Fig. 14. Left valve of Cytherea chione* 



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

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

 impression {p ) ; the presence of a bay, or sitius (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 animal is withdrawn by retractor muscles also attached to the 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 withdi'aw into their shells w^hen 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^ concJiolepas, and the nautilus ; it be- 



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

 hgament ; u, the umbo ; I, the lunnle ; c, cardinal tooth ; 1 1\ lateral teeth ; 

 a, anterior adductor ; a\ posterior adductor ; f, pallial impression ; s, sinus, 

 occupied by retractor of the siphons. 



