92 



NATURAL HISTORY. 



axes and war-clubs, all of a delicate creamy white color, and considerable 

 hardness Some of these axes will weigh seven or eight pounds. Mr. 

 Forbes in his account of the Cocoa-Keeling Islands, speaks of "the giant 

 clams of the genus Tridacna, whose mantle, edged with turquoise beads,, 

 forms a beautiful object to look down upon ; but one must shudder for the 

 diver who should accidentally thrust his head or limb into its gape, which 

 the slightest touch causes to close with a snap." He also had evidence 

 that one individual reached a length of twelve and a breadth of ten inches 



in three years. 



Some of the heart-shells, or cockles (Cardium), are beautiful, both m 

 shape and color, while others are less attractive. Some are good for food, 

 and are eaten to a considerable extent in Europe and the East. In these 

 forms the foot is large and peculiarly shaped, as is shown in the cut, It is, 



Fig. S4. — C'ockle-skell {Cardium) pulling itself along by its muscular foot. 



in fact as well as in name, an organ of locomotion. The cockle will at 

 times hook it around some stone or shell, and with this anchorage warp 

 itself along; or. again, it will extend it to its full length and then, by sud- 

 denly 1 lending it, will throw himself forward and over and over. 



Next to the oyster and clam, the quahog (Venus mercenaria) is our 

 mosl valuable shell-fish. It is distributed in abundance all along our 

 eastern coast, from Cape Cod to Florida, and more rarely north of those 

 limits. It frequents sandy bottoms, crawling along on the surface or bur- 

 rowing just below. In edible qualities it is inferior to either of the forms 

 mentioned above, but the length of time it will live after removal from its 

 native element renders it a valuable species for shipment to the interior. 

 Indeed, a few hundred miles from the shore it is the clam, and the far 

 better My a is absolutely unknown. 



