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 is a fragile V-shaped car- 

 dinal tooth, which is generally broken away in forcing open the valves ; 

 the lateral teetb 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 simila_ 

 outline, but less heavy and strong. Its range is from Hatteras south- 

 ward to Brazil. Passing from New Jersey to Hatteras, Mactra sol' 

 dissima diminishes in size, and after passing that faunal barrier thv. 

 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 

 length. It is white, and is covered with a dirty 

 brown epidermis j the hinge is strong, with deep Mactra fa(m(Hgi 

 cartilage fossette and a V-shaped marginal tooth. 

 The laterals are long and slender. The animal has a strong foot 

 and two slender, long siphons which are yellow in color. 



GENUS Rceta 



M. (Labiosa) canaliculate,. In this species the cartilage fossette 

 is the same as in Mactra ; the marginal teeth are feeble, and there are no 

 laterals. The shells of Mactra which we have examined are heavy o r * i 

 least solid, but Rteta is thin and fragile ; it is pure white in color anu ;s 

 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 external ribs. 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 VENERIDJE 



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 



