AKCA. 321 



number, in each valve ; the teeth of one valve are closely and 

 curiously interlocked with those of the opposite valve, and 

 the hinge or cardinal axis, thus constituted, runs through 

 the centre of the ligamentary area. 



The genus is divided into two very distinct groujTs or 

 sections, and various modifications, either as regards their 

 composition, growth, or sculpture, arise from difference of 

 habits in the occupants. 



Such as pertain to the first division live free and unat- 

 tached, their shells are mostly solid and ponderous, uni- 

 formly disposed in strongly-developed ribs radiating from 

 the umboes, and giving a crenulated margin to the valves, 

 by which they interlock one with the other. The shell is 

 often inequivalve, the margin of the left valve extending 

 beyond that of the right, and generally exhibiting a more 

 elaborate style of sculpture, as if the left mantle of its resi- 

 dent was more highly organized than the right. 



The second division live attached to rocks and stones, by 

 means of a strong muscular byssus, inserted through an 

 opening in the margin of each valve ; the shell is of a lighter 

 structure and composition, and, being seldom distinctly 

 ribbed, the margin of the valves is not crenulated. The 

 protection Avhicli the byssus-bearing Arcce gain by their 



Y 



