PELECYPODS 447 
beaches. It is covered with a thin brownish or straw-colored epidermis, 
usually more or less worn away in adult specimens and wholly absent from 
the dead valves found upon the beaches. There isa fragile V-shaped car- 
dinal tooth, which is generally broken away in forcing open the valves; 
the lateral teeth are long and thin, and striated on their receiving sur- 
faces. The length of this “giant clam” is from four to seven inches. 
This is the first shell the collector will find (north of Hatteras) when he 
goes to an open, sandy beach. 
M. similis. A small variety of the species last described, of similar 
outline, but less heavy and strong. Its range is from Hatteras south- 
ward to Brazil. Passing from New Jersey to Hatteras, Mactra soli- 
dissima diminishes in size, and after passing that faunal barrier the 
Northern form is entirely replaced by this Southern variety. The shell 
is brighter, though not exactly bright-shining. Its habits are entirely 
similar to those of the Northern form. On the west coast of Flor- 
ida it is the most abundant bivalve upon the beaches. 
M. ovalis. A species of Northern range, which so greatly resembles 
M. solidissima as to suggest identity; but the lateral teeth are smooth 
and not striated, while the shell itself seldom attains 
a length of over four inches. 
M. lateralis. An exceedingly common shell, 
found in all the salt-marshes and tidal estuaries of 
our coast. It is much smaller than the mactras just 
described, being not over one half of an inch in 
Paget It is epee and is covered with 2 oy — 
rown epidermis; the hinge is strong, with deep alain alia: 
cartilage fossette and a V-shaped marginal tooth. seamen 
The laterals are long and slender. The animal has a strong foot 
andtwo slender, long siphons which are yellow in color. 
Genus Reta 
R. (Labiosa) canaliculata. In this species the cartilage fossette 
is the same asin Mactra ; the marginal teeth are feeble, and there are no 
laterals. The shells of Mactra which we have examined are heavy or at 
least solid, but Reta is thin and fragile ; it is pure white in color and is 
gracefully ornamented by concentric raised ribs. The shell is inflated 
and gapes slightly posteriorly ; it is shining white within and is deeply 
grooved concentrically in accordance with the externalribs. Length two 
and a half, height two inches. From Hatteras to Mexico, this is one of 
the commoner beach shells, but since, as in some other species already 
named, the hinge ligament and teeth are feeble, the valves become 
separated by the buffeting of the waves, and are cast disjointed and 
singly upon the beaches. (Plate LXXXI.) 
FAMILY VENERIDE 
In many respects this is the most distinguished pelecypod fam- 
ily. ‘It is the largest in number of genera and species, is of 
world-wide distribution, and supplies to hungry man a number 
