184 



ZOOLOGY 



its name " mercenaria " to the fact that the purple-colored 

 patch seen on the margin of the shell was used as money 



FIG. 170. Venus mercenaria, the hard-shelled clam. At the left the foot 

 protrudes from the shell ; at the right the siphons. Nat. size. From Verrill, 

 " Invertebrates of Vineyard Sound." 



(" wampum ") by the Indians. The shell is extremely heavy, 

 and usually nearly circular in outline (Fig. 170). 



The Arcidae * and their allies include several species hav- 

 ing a more or less elongated 

 hinge -joint crowded with 

 many small teeth. Our com- 

 mon Eastern species (Area 

 pexata, Fig. 171) has bright 

 red blood and is commonly 

 known as the "bloody clam." 

 The Mytilidae 2 include 



FIG. 171. Area pexata, the bloody J 



clam. Nat. size. Photo, by the mussels, which are the 



W. H. C. P. 



1 area, an ark. 



2 /iUTi'Xos, an edible mussel of the Greeks. 



