BIVALVES. — ARCA. 33 



ARCA.— Ark. 



Animal— a Tethys? Shell bivalve, equivalve; hinge with 

 numerous sharp teeth, alternateli/ inserted between each 

 other. 



The Area, of which there are forty-three species, are rea- 

 dily distinguished from the other Bivalves, by the peculi- 

 arity of the hinge ; which, without exception, is composed 

 of numerous sharp teeth, alternately inserted into or be- 

 tween each other. The line of direction of the hinge 

 admits, however, of two variations: in some species, as 

 the Area Nbae, Barbata, &c. it is perfectly straight ; in 

 others, as the Area Pectunculus, Pilosa, &c. &c. it is 

 arched or curved. However, in all the species, the same 

 description of hinge exists, though in some it may be dif- 

 ferently situated. 



The forms of the Arks vary exceedingly ; some are 

 elongated, as the Tortuosa, Noae, and Barbata, and are 

 usually covered with a brown epidermis; in the Barbata 

 or Bearded Ark, the greater part of the surface of the shell 

 is concealed by a thick bristly or hairy covering. There 

 are many Arks which gape at the outer margin; others, 

 on the contrary, are perfectly close. Some have the 

 margin entire ; others, again, are crenulate, and several 

 have prominent angular slopes, which give the shell an 

 appearance of being eared ; the anterior slope is, how- 

 ever, far the most prominent of the tw o. The next vari- 

 ation of form is manifest in the Area Senilis, Granosa, and 

 Rhomboidea, which rather partake of the shape of a heart, 

 than of any other fonn. They are, besides, somewhat 

 gibbous, and usually covered with smooth or muricate 



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