170 ARCID^. 



as in the last genus ; but the bundles are not in all cases 

 placed diagonally, being in some species across the space be- 

 tween the beaks : hinge-line straight : teeth laminar, and set at 

 or a little more or less than a right angle to the hinge-line, con- 

 tinuous in most species, but divided into two distinct rows in 

 others : pallial scar entire : muscular scars oblong, symmetri- 

 cal and strongly marked. 



Area rivals Pectunculus in its high descent, as well 

 as in its fertility; and it far excels it in the nnmber 

 of species. It has, besides, a wider distribution, one 

 species {A. pectunculoides) being found both on the coast 

 of Greenland and in the iEgean. Its bathymetrical 

 range is considerable, extending from low-water mark 

 to between 300 and 400 fathoms. Unlike Pectunculus, 

 it spins a byssus, and is by this means attached to sub- 

 marine substances, from which it can disengage itself 

 at pleasure ; or it takes permanent shelter and makes its 

 abode in the cranny of a rock, the shell occasionally 

 becoming distorted by the narrow limits of its habi- 

 tation. 



The generic name is encumbered by a load of no less 

 than thirty synonyms, which have been from time to 

 time imposed by the fanciful ambition of systematists, 

 or for the more laudable purpose of distinguishing parti- 

 cular groups of species. It will probably always remain 

 a question whether subgenera are advisable, the settle- 

 ment of it depending in a great measure on the defini- 

 tion of a genus. The characters of all genera cannot be 

 equivalent ; or perhaps we have not yet found the right 

 key to Nature's lock. All are agreed as to the existence 

 of varieties ; and they must be discriminated by certain 

 names, in the same way as species, genera, and higher 

 groups. One of Linne's botanical axioms may be cited 

 in support of this method of identification : — " Nomina 

 si nescis, perit et cognitio rerum." Whether subgenera 



