446 MARINE INVERTEBRATES 
If they were shells of arctic range 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 MACTRIDE 
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 
Mactra sotidissima. Island, and New Jersey | 
