36 LINNEAN GENERA. 



plate VI. fig. 12. (Isorcardia Cor, Lamarck.) Shell sub -globose, 

 beaks recurved ; anterior slope with a gaping fent, slightly 

 wrinkled transversely, covered with a terra-sienna coloured 

 epidermis ; umbo large and prominent, much contorted or incur- 

 vated, 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 ; and has been found 

 off the North Foreland, the Hebrides, and in the Irish Channel 

 of large size. 



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



The shells of this geuus inhabit the ocean j they live only In very deep 

 water. 



The Chama Gigas, (Tridacna Gigas, Lamarck,) Giant Chama, is the largest 

 and heaviest shell yet discovered, being sometimes of the amazing weight 

 of 53S pounds, and the animal inhabitant so large as to furnish one hundred 

 and twenty-two men with a meal ; it is said to be very palatable. Such 

 is its strength that it is able to cut asunder a ship's-cable. This species in- 

 habits the Indian Ocean. 



Genus 13.— ARC A. 



Animal a Tethys ', shell bivalve, equivalve ; hinge with 

 numerous sharp teeth, alternately inserted between each other. 



Area NocB. — Noah's Ark. 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 ; orifices 

 brown; and margin crenated. Two inches broad. Inhabits the 

 West Indian seas ; also Britain — a small size. 



Linne divides this genus into two families : * with the margin entire ; 

 ** with margin crenulatcd ; and has subdivided them as follows : f beaks 

 recurved ; and ff beaks inflected. 



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

 others lurk 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 furnished with an 

 ovate hollow, and for the most part lateral transverse grooves. 



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

 8s in scallops ; ** rugged or rough, as in the oyster ; *** hinge with a per. 

 pondicular furrowed line, as in the species of the genus Perna. The first family 

 is again subdivided into three sections : f valves equilateral, and ears e<iual ; 



