THE LAMELLIBRAXCIIIATA. 



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:.Tani of a CrPsh-waterM.iss -I { .\iiorlori^ri).—A. ^.mantle, the 

 right lobe of which is cut away; B. font : 6'. branchial cliamber of the mantle 

 cavity; T). anal chamber; /, anterior adductor miiscl=': IT, posterior adductor 

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

 the turns of which are supi)osed to be seen throuirh the side walls of the meso- 

 soma; d. rectum; e, anus: /', ventricle; q, auricle; /?, jrills, except i. riirht exter- 

 nal ffill, larirely cut away and turned back; l\ labial palpi ; I, cerebral ; ra. pedal; 

 n, parieto-splanchnic ganglia; o, aperture of the kidney or or^an of Bojanus; J9, 

 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 svstematic conchology. 



