52 INVERTEBRATA OF MASSACHUSETTS. 



Mactra gigantea, LAM. ; An. sans Vert., vi. 97. DESHAYES ; Eticyc. Mtth., Vcrs, 



ii. 394, pi. 259, f. 1. 

 Mactra luteola, LOVEN ; Mss. (young.) 



Shell very large and solid, transversely ovate, somewhat trian- 

 gular, nearly equilateral, anteriorly a little the shortest, and with a 

 fissure between the beaks ; surface slightly folded at the lines of 

 growth, covered with a thin, dirty-brown or straw-colored epider- 

 mis, which is usually worn from the disc ; beaks large and pro- 

 tuberant, inclined a little forwards ; behind them is a broad, 

 lanceolate space, bounded by sharp ridges passing from the beaks 

 to the upper part of the basal angle, in which the epidermis is 

 foliaceous, or very loosely wrinkled ; a much more faintly de- 

 veloped areola may also be observed before the beaks. Hinge 

 very strong, spoon-shaped cavity very large and broad, the V 

 tooth very delicate, and adhering by a very small base, so that it 

 is usually broken off in the cartilage ; lateral teeth long and thin, 

 striated on their receiving surfaces. Muscular and palleal im- 

 pressions very decided, the posterior sinus of the latter quite 

 shallow. Length 4| inches, height 3 inches, breadth If inch. 



It is found about sandy beaches on all our coast, and its in- 

 habitant is much esteemed by some, as an article of food. At 

 low water it is dug out of the sand with shovels. At higher tides, 

 when the shell is open, the fishermen wade into the water, 

 thrusting a pointed stick into the sand before them, as they walk 

 along. If the stick happens to pass between the valves, they are 

 closed upon it by the animal, and the shell is thus drawn up. 



This is sufficiently characterized at maturity by its great magnitude, 

 which entitles it to the name of giant clam. No other species of the 

 genus approaches it in size except the next, from which it may be dis- 

 tinguished at all stages by its striated lateral teeth. The largest 

 specimen I have seen measures 6^ inches in length, 4 in height, and 

 2-I- in breadth. It preserves its general features through all ages, ex- 

 cept that, when young, it is more nearly equilateral, and, after it has 

 attained its full dimensions, it acquires great weight and thickness. 

 Some specimens obtained in Provincetown harbour, where there are 

 no rocks and little surf, are very light, thin, and white. 



A specimen from the cabinet of Colonel Totten, which he found at 

 Newport, I presume to be the M. similis of Say. Whether it be sim- 



