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. / 



Pio. 117.— Sectional diagram of a Fresh- water Mass -1 (Anodonia). — A, A. mantle, the 

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

 cavity; D. anal chamber; 7, anterior adductor muscle; IF, 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 wall* of the meso- 

 soma; d, rectum: e, anus: /, ventricle; g, auricle; h, ffills, except i, riirht exter- 

 nal sfill, largely cut away and turned back; k, labial palpi ; /, cerebral; m, pedal; 

 n, parieto-splanchnic ganglia; o, aperture of the kidney or or<>an 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. 



