42 LINN^AN GENERA. 



slightly wrinkled transversely, covered with a terra-sienna- 

 coloured epidermis ; umbo large and prominent, much con- 

 torted or incurvated, turned to one side, and not touching 

 when the valves are closed ; hinge strong, teeth flattish ; 

 inside smooth, white , margin plain. This is a very scarce 

 British shell; but has been found oif the North Foreland, 

 the Hebrides, and in the Irish Channel, of large size. 



Disk, r r beak. Plate II. fig. 5. 



The shells of this genus inhabit the ocean ; they live only in very 

 deep water. 



The Chama Gigas, (Tridaena Gigas, Lamarck,) Giant Chama, is the 

 largest and heaviest shell yet discovered, an individual having been 

 seen of the amazing weight of 532 pounds, and the animal inhabitant 

 so large as to fui-nish one hundred and twenty-two men with a meal ; 

 it is said to be very palatable. This species inhabits the Indian Ocean. 



Genus 13 ARCA. 



Animal a Tethys; shell bivalve, equivalve; hinge with 

 numerous sharp teeth, alternately inserted between each 

 other. 



tlrca Noce Noah's Akr. Plate VI. fig. 12. Shell rhom- 

 bic, reticulated, beaks near one end, incurved, very remote 

 and separated by a deep groove ; of a deep umber colour, or 

 whitish, with brown bands; orifices brown; and margin cre- 

 nated. Two inches broad. Inhabits the West Indian seas, 

 the Mediterranean and also Britain, but there of a small size. 



Linnaeus divides this genus into two families : * with the margin 

 entire ; ** with margin crenulated ; and has subdivided them as fol- 

 lows : t beaks recm-ved ; and j t beaks inflected. 



Some of the Arcae are affixed by the foot of the animal to rocks ; and 

 others lui-k in the sand or mud near the sea-shore, generally between 

 high and low water-mark. 



Genus 14 OSTREA. 



Animal a Tethys ; shell bivalve, generally with unequal 

 valves, and slightly eared; hinge without teeth, but fur- 

 nished with an ovate hollow, and for the most part lateral 

 transverse grooves. 



The Ostreae are divided into three families: * valves radiated and 

 eared, as in scallops ; ** rugged or rough, as in the oyster ; *** hinge 



