ARCID^. 139 



attached by a solid byssus, may escape this doom for a 

 longer period. The thickness and apparent longevity of 

 specimens of A. tetragona and A. lactea may be thus 

 accounted for. Lamarck proposed the expressive name 

 of " Polyodontes " for the present family. According 

 to Dr. Carpenter, a microscopical examination of the 

 shell shows in many of the genera a tubular structure 

 intersecting the upper layer and spreading outwards, the 

 lower or inside layer being nacreous. 



The Arcidce may be conveniently divided into two 

 sections, viz. 



t Shell triangular or wedge-shaped : epidermis smooth and 

 glossy : beaks close together and recurved : hinge fur- 

 nished with a row of teeth set like those of a comb, on 

 each side of an oblique spoon-shaped process, which 

 contains a cartilage. 



* Shell triangular. 



1. Nucula. 



** Shell wedge-shaped. 



2. Leda. 



ft Shell round, oblong, or rhomboidal: epidermis hairy or 

 fibrous : beaks separated by the ligamental area, and in- 

 curved : hinge furnished with a single continuous row 

 of plate-like teeth, destitute of an intermediate cartilage 

 or pit. 



* Shell round. 



3. Limopsis. 



4. Pectukctjlus. 



** Shell oblong, or rhomboidal. 

 o. Aeca. 



