446 MARINE INVERTEBRATES 



If they were shells of arctic i-ange it would be easier to connect them ; 

 but as neither is a cold-water form, it becomes more difficult to place 

 them under one name. The increasing number of similar species being 

 found upon the Atlantic and Pacific sides of Central America points 

 almost conclusively to the existence of a waterway between the two 

 oceans, somewhere between North and South America, at no very remote 

 geological period. Possibly the closing of some such waterway through 

 the isthmus connecting the oceans has separated these forms geographi- 

 cally, thus leading us to separate them specifically. (Plate LXXXI.) 



FAMILY MACTRIDJE 



This is a very large family of universal distribution and with 

 a great number of genera and subgenera, rather confusedly 

 gathered into subfamilies. The dominant genus Mactra comprises 

 the largest bivalve upon the east coast of the United States. 



GENUS Mactra 



The mactras live in the sand near the margin of the water, 

 often upon the exposed open coast. The mantle is open, except 

 where it is fused to form the siphons. These are short and 

 united. The outer gill is dorsally directed ; that is to say, it does 

 not depend into the mantle cavity as does the inner gill, but 

 projects just the other way; it is also smaller than the inner one. 

 The foot is strong, bent, and tongue-shaped, and no doubt well 

 adapted to the rough life in the surf and the heavy, shifting 

 sands. The main characteristic of the mactra shell is the 

 prominent triangular-shaped fossette, or cartilage plate, situated 

 internally and just under the beaks. The shells are equivalve, 



and devoid of bright 

 colors or striking 

 sculptural features. 

 Internally the pallial 

 line is plain, and the 

 sinus well marked but 

 not deep. 



M. solidissima is 



one of the very com- 

 monest, if not the com- 

 monest large bivalve of 

 the New England, Long 

 Island, and New Jersey 



