38 GENERA OF SHELLS. 



lar folds, the innermost very small, and has the appear- 

 ance of a bundle of very unequal, diverging laminae. 

 This compound tooth is rather depressed, and always 

 inclines towards the posterior side of the shell, instead 

 of rising perpendicularly to the plane of the valve. 

 H, avicularis, corrugala.* 



Anodonta. Shell equivalve, inequilateral, 

 transverse. Hinge linear, toothless ; a cardi- 

 nal lamina, smooth, adnate, truncated, or form- 

 ing a sinus at its anterior extremity, termina- 

 ting the base of the shell ; two muscular im- 

 pressions, remote, lateral, subgeminate ; liga- 

 ment external, linear, descending at its anterior 

 extremity into the sinus of the cardinal lamina. 



The anodontae are fresh water shells, usually very 

 thin, and often of a large size. They greatly resemble 

 the unio, but have neither cardinal nor lateral tooth, 

 the hinge presenting merely a smooth interior margin 

 or laminae, situated immediately below the nymphse, 

 and terminated anteriorly by a trmication or sinus. 

 The shell is pearly, and covered Avith a thin, greenish, 

 false epidermis ; beaks oblique, partly inclining to the 

 posterior margin. 



*Symphynota.— Saij. Shell fluviatile, bivalve ; valves con- 

 nate at the dorsal margui. 



S. lEBvissima, bialata, alata, complanata, compressa, gracilis, tenuis- 

 sima, ochracea, cygnea. 



Ai-ASMiDONTA. — Saij. Shell transverse, equivalve, inequila- 

 teral J hinge with a primary tooth in each valve j cicatrices three. 



It is separated from anodonta by its primary tooth, and from 

 unio, by being destitute of the lamelliform teeth. 

 A. undulata, marginata, ambigua, confragosa. 



