FAMILY SAXICAVID.E PETRICOLA. 229 



This species has been found at Glasshouse point above the city, and is a more robust shell 

 than the preceding, from which it differs chiefly in the want of a definite area before the 

 beaks. It appears to range from Massachusetts to Carolina, but is more rare than the pre- 

 ceding. 



FAMILY MACTRIDJE. 



Shell equivalve, frequently gaping at the sides. Hinge with an internal ligament, and 

 sometimes an external ligament beside. Animal with a small foot, but well adapted for 

 motion. 



GENUS MACTRA. Lamarck. 



Animal with the edges of the mantle thickened and simple, furnished behind with two united 

 moderately long tubes. Mouth small ; labial appendages narrow and pointed. Branchial 

 plates small, and nearly equal. Foot oval, trenchant, very long and angular. Shell 

 transverse, slightly gaping at the sides : beaks prominent. Hinge a prostrate concave 

 tooth to contain the cartilage, having at one margin a delicate erect tooth, like the letter v : 

 two lateral teeth near the central ones. 



Mactra solidissima. 



PLATE XXIX FIG 286. 



(STATE COLLECTION.) 



Mactra solidissima. Chemnitz, Conch. Vol. 10, p. 365, pi. 170, fig. 1656. 



M. id. Conrad, Am. Mar Conchology, 64, pi. 14, fig, 7. Id Ac. Sc. Vol. 6, p. 257. 



M. gigantea. Lamarck, An. sans vert. Ed. Brux. Vol. 2, p. 535. 



Hipissula. Gray, Loud. Mtg. Nat. Hist. New series, Vol. 1, p. 373. 



M. solidissima. Gould, Invertebrata of Massachusetts, p. 51. 



Description. Shell large and solid, subtriangular, nearly equilateral, smooth or very slightly 

 wrinkled by the lines of growth. Beaks large and protuberant, directed slightly forwards ; 

 nearly central, and behind them a broad somewhat flattened space bounded by a rounded 

 elevation from the beaks. Hinge very strong ; the spoon-shaped cavity large ; the v tooth 

 very delicate, and adhering by a very small base, so that it is usually broken off in the car- 

 tilage ; lateral teeth long and thin, and regularly striated on the side next the recipient cavity. 



Color. Epidermis thin and olive-brown or light yellowish ; beneath this, chalky white. 



Vertical axis, 1*5 — 4*5; transverse ditto, 2'0-6*0. Diameter, l'0-2 - 5. 



