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PARTS OF THE SHELL IN BIVALVES 



CHAP. 



impressions. The impression produced by the muscular edge of 

 the mantle, which curves downwards and backwards from the 

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

 shells with only one muscle it is represented by an irregular 

 row of small marks, or disappears altogether QOstred). The 

 pallial 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 A^alve of Venus gnidia L. : 

 A, anterior, B, posterior, C, dorsal, 

 D, ventral margin, AB, length, CD, 

 breadth of shell. 



a.m, anterior ; p.wi, posterior adduc- 

 tor muscle; p, pallial line; p.s, pallial 

 sinus ; I, ligament ; lu, lunule ; u, umbo ; 

 c, cardinal teeth ; a.l, anterior lateral 

 tooth; p.l, posterior lateral tooth. 



Fig. 185. — Right valve of Lucina tigerina 

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

 D, ventral margin ; AB, length, CD, 

 breadth of shell. 



a.m, anterior; p.m, posterior ad- 

 ductor muscles; p, pallial line; I, liga- 

 ment ; II, umbo ; c, cardinal teeth ; a.l, 

 p.l, anterior and posterior lateral 

 tooth. 



Right ayid 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 valve. If, however, 

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

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

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

 be named by observation of the beaks and ligament. When 

 the ligament is median to the valves (e.g. Ostrea^ Fecten'), and the 

 beaks are not curved, the valves mav be recognised by noting 

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



