BIVALVES— ARCA. 49 



ARCA.— Ark. 



Animal— a Tcthyi? Shell bivalve, equivahe; hinge with 

 numerous sharp teeth, alternately inserted betiveen «ach 

 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 Noee, 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 arc 

 elongated, as the tortuosa. Nose, 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, however, far the most prominent of the two. The 

 next variation of form is manifest in the Area senilis, 

 granosa, and rhomboidea, which rather partake of the 

 shape of a heart, than of any other form. They are, 

 besides; somewhat gibbous, and usually covered with 



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