THE LAMELLIBRANCHIATA. 



407 



face is usually marked by concentric lines of growth, which 

 indicate the boundaries of the successively added new layers 

 of shell-substance. 



Pig. 117.— Sectional diagram ofa fresh- water Mnssel (Anoclonta).— A, J., mantle, the 

 light lobe of which is cut away; B, foot; C, branchial chamber of the mantle 

 cavity; D. anal chamber; I, anterior adductor muscle; II, posterior adductor 

 muscle; III retractor muscle of the foot; a, mouth; b, stomach; c, intestine, 

 the turns of which are supposed to be seen through the side-walls of the meso- 

 soma; d, rectum: e, anus; /, ventricle; q, auricle; A, gills, except ?'. riuht exter- 

 nal gill, largely cut away and turned back; k, labial palpi; I, cerebral; m, pedal; 

 n. parieto-splanchnic ganglia; o, aperture of the kidney or orsran of Bojanus; p, 

 pericardium. 



The applied edges of the two valves are very often pro- 

 duced into elevations and depressions which interlock with 

 one another. The form and arrangement of these teeth and 

 sockets are of much use in systematic conchology. 



