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. 



Fia. 117.— Sectional diasrram of a fresh-water Mussel { Anodonta).—A^ ^.mantle, the 

 right lobe of which is cut away; jB, foot,; C, branchial cliamber of the mantle 

 cavity; D. anal chamber; /, anterior adductor muscle; 77, posterior adductor 

 muscle; 777, retractor muscle of the foot; «, mouth; 6, stomach; c, intestine, 

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

 eoma; d, rectum: e, anus; /, ventricle; g, auricle; A, ffills, except i. rieht exter- 

 nal orill, lar<rely cut away and turned back; A-, labial palpi; /, cerebral: m, pedal; 

 n. parieto-splanchnic ganglia; o, aperture of the kidney or organ of Bojanus; i?, 

 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. 



