2/0 



PARTS OF THE SHELL IN BIVALVES 



imjyressioois. The impression produced by the muscular edge of 

 the mantle, which ciu-ves downwards and backwards from the 

 anterior adductor impression, is known as the imllial line. In 

 shells with only one muscle it is represented by an irregidar row 

 of small marks, or disappears altogether {Ostrect). The pfdlial 

 sinus is produced by the muscles which retract the siphons, 

 and is most marked in those genera in which the muscles are 

 powerful and the siphons large (e.g. Tellina, My a). It is entirely 

 absent in genera possessing no retractile siphons. 



Fig. 184 — Left valve of Venus gnidia L. : 

 A, anterior, B, i:)Osterior, C, dorsal, 

 D, ventral margin, AB, length, CD, 

 breadth of shell. 



a.vi, anterior ; ^). 771., posterior adductor 

 muscle ; 2h pallial line \ p.s, pallial sinus; 

 I, ligament ; hi, lunule ; u, umbo ; c, 

 cardinal teeth ; a. I, anterior lateral tooth ; 

 2^.1; posterior lateral tooth. 



Fig. 18.5. — Eight valve of Lucina tigerina 

 L. : A, anterior, B, posterior, C, dorsal, 

 D, ventral margin ; AB, length, CD, 

 breadth of shell. 



a. m, anterior ; }). in, posterior adductor 

 muscles ; p, pallial line ; /, ligament ; n, 

 umbo ; c, cardinal teeth : a. I, p.l, ante- 

 rior and jiosterior lateral tooth. 



Right and Left Valve. — The simplest way of distinguishing the 

 valves as right and left is to hold the shell in such a way that 

 the siphons point towards the observer, and the mouth away 

 from him ; in this position the valve to the right is called the 

 right valve, and the valve to the left the left vcdve. If, however, 

 the animal is not present, it may be remembered that the liga- 

 ment is nearly always hehi7id the beaks, and that the beaks, as a 

 rule, point forward, thus the right and left valves can generally 

 l)e named by observation of the Ijeaks and ligament. When the 

 ligament is median to the valves (e.g. Ostrea, Pecten), and the 

 Ijeaks are not curved, the valves may be recognised by noting the 

 fact that the impression of the adductor muscle (in these cases 



