58 BIVALVES. — ARCA. 



ARCA.— Ark. 



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



numerous sharp teeth, alternately inserted hetweeji each 



other. 



THE Area, of which there are forty-five species, are 

 readily distinguished from other bivalves, by the peculiari- 

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

 numerous sharp teeth, alternately inserted between each 

 other. The line of direction of the hinge admits of two 

 variations: in some species, as the A. Noaa, A. barbata, 

 &c. it is perfectly straight j in others, as the A. pectuncu- 

 lus, 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 elong- 

 ated, 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. 



The 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- 

 cymeris and A. pilosa are sufficient examples; they are for 

 the most part smooth on the outside, except where the 

 longitudinal strias are placed: these strias are fi-equently 

 more articulate in the interior than on the exterior, and of- 

 ten terminate, at the inner margin, in "elevated teeth. The 

 shells of this genus are covered with a brownish or green- 

 ish black epidermis. 



There are many Arks which gape at the outer margin ; 

 others, on the contrary, are perfectly close. Some have 



