56 BIVALVES— ARCA. 



ARC A.— Ark. 



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

 numerous sharp teeth, alternately inserted between each 

 other. 



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

 readily distinguished from other bivalves, by the pecu- 

 liarity of the hinge; which, without exception, is com- 

 posed of numerous sharp teeth, alternately inserted be- 

 tween each other. The line of direction of the hinge ad- 

 mits of two variations : in some species, as the A. Noae, 

 A. barbata, &c. it is perfectly straight; in others, as the 

 A. pectunculus, A. pilosa, &c. it is arched or curved. — 

 However, in all the species, the same description of hinge 

 exists, though in some it may be differently situated. 



The forms of the Arks vary exceedingly; some are 

 elongated, as the A. tortuosa, A. Noae, and A. barbata: 

 in the A. barbata or bearded Ark, the greater part of the 

 surface of the shell is concealed by a thick bristly or 

 hairy covering. 



Tiie subcordate form is exemplified in the A, senilis, 

 A.granosa, &c. which are, besides, somewhat gibbous, 

 and have usually smooth or muricated grooves. 



The third form is the suborbicular, of which the A. gly- 

 cimeris and A.pilosa are sufficient examples; they are 

 for the most part smooth on the outside, except where 

 the longitudinal striae are placed: these striae are fre- 

 quently more articulate in the interior than on the exte- 

 rior, and often terminate, at the inner margin, in deter- 

 mined elevated teeth. The shells of this genus are co- 

 vered with a brownish or greenish-black epidermis. 



