336 INTRODUCTION TO CONCHOLOGY. 



The genus is provided with a comparatively large shell, 

 easily recognized by its thick fibrous epidermis. 



The shell of the Astarte is nearly orbicular, and covered 

 with a brown epidermis. Two divergent teeth pertain to 

 the right valve of the hinge — one distinct, and one obsolete 

 tooth to the left. 



The shell of the Venus is equivalve, inequilateral, nearly 

 orbicular, subglobose, generally rough on the outside, and 

 closely shut. The cardinal teeth mostly belong to each 

 valve, somewliat contiguous, and diverging from the umbo. 



Three divaricate cardinal teeth are obvious in one valve 

 of the Cytherea, with a somewhat remote pit running 

 parallel with the margin ; and four in the other, one remote, 

 diverging laterally, and almost considered as a lateral tooth, 

 inserted in the pit of the opposite valve. 



Tlie equivalve, transverse, and equilateral shell of the 

 genus Ptdlastra, which is much shorter on the anterior 

 side, has, also, three contiguous cardinal teeth, sometimes 

 a little emarginated at -the point in each valve, but no 

 lateral teeth. The muscular impression of the mantle has 

 a large sinus, and the ligament is external, though partially 

 concealed. 



The component parts or superficies of shells are often 



