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 cytJierea (and other di-myaries), by two ; 

 the contraction of which brings the valves together. They are 

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

 are attached is always indicated by scars. (Fig. 14, a. a!) 



h 



1 



Fig. 14. Left valve of CytTierea 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 (^ ) ; the presence of a bay, or sinus ( « ), in this Hue, | 

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

 animal is withdrawn by retractor muscles also attached to the 

 shell, and leavins: 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, concJiolepas, and the nautilus ; it be- 



* Fig. 14. Ci/tJierea chione, L., coast of Devon, (original) ; h, tte hinge 

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

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

 occupied by retractor of the siphons. 



