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. 



Fig. 117.— Sectional dia^raiu of a fresh-water Mussel {Aao(lonta)i.—A^ ^.mantle, the 

 risfht lob<* of which is cut away; B, foot; C, hranchial chamber of the mantle 

 cavity; T>. anal chamber; /, anterior adductor muscle; //, posterior adductor 

 muscle; ///.retractor muscle of the foot; «, mouth; 6, stomach; c, intestine, 

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

 eoma; d. rectum: e, anus; /; ventricle; q, auricle; /?, gills, except ?, right exter- 

 nal gill, largely cut away and turned back; A', labial palpi ; ^, cerebral: m, pedal; 

 n, parieto-splauchnic ganglia; o, aperture of the kidney or organ 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. 

 18 



