THE LAMELLIBRANCHIA 51 



as in brachiopods. The edges of these mantle-folds are 

 thickened, both because this is the region by which new 

 shell is secreted, and because there is here an important 

 muscle (orbicular muscle) by which the lips of the mantle 

 can be pressed together even when the valves are not 

 closed. The pallial line marks the inner margin of this 

 thickening of the mantle. The mantle-edges separate 

 only at certain points (i) antero-ventrally, where the 

 foot may be protruded ; (2) posteriorly, where they enclose 

 two openings like a figure 8 : the lower opening being 

 that by which water is sucked in (inhalent aperture), the 

 upper that by which it is expelled (exhalent). These two 

 apertures are in some lamellibranchs extended into long 

 projecting siphons, and in that case the necessary local 

 increase in the orbicular muscle, to form a retractor for 

 the siphons, causes the pallial line to be more or less 

 indented as it approaches the posterior adductor. Such 

 an indentation is termed a pallial sinus : it is not present 

 in Pectunculus. 



It is necessary now to point out which of the characters 

 seen in the shell of Pectunculus are not characteristic of 

 all lamellibranchs, and for these technical terms must be 

 given. 



The shell of Pectunculus, having the umbo directed 

 towards the hind end, is opisthogyral. The elastic ligament, 

 as it lies entirely dorsal to the hinge-line, is an external 

 ligament or tension-ligament. As it extends without inter- 

 ruption fore and aft of the umbo, it is amphidetic. The 

 same adjective may be applied to the cardinal area. The 

 hinge, being set with numerous undifferentiated teeth, is 



