72 • MACTRAD.E. 



Thracia truncata. 



Shell small, rather solid, beaks posterior, broadly truncate behind, hinge-pit 

 elongated. 



Thracia truncata, Migh. and Adams, Bost. Journ. Nat. Hist. iv. 38, pi. 4, fig. 1 (1842). 

 — Stimpson, Shells of New England, 22 (18.51); Mar. Invert. Gr. Manan, 21 

 (1853) ; Check Lists, Smith. Inst. 3. — Mouch, GriJnl. Bluddyr, 18 (1857). 



Shell small, ovate-triangular, compressed, white, rather solid ; 

 beaks at posterior fourth, anterior disk semi-oval, posterior trian- 

 gular, abruptly truncate at tip, posterior dorsal margin rapidly slop- 

 ing ; a ridge passes from the beaks to the posterior 

 ventral angle ; anterior dorsal margin nearly hori- 

 zontal ; surface with numerous and conspicuous lines 

 of growth, and covered by a pale yellowish epider- 

 T. truncata. mis ; bcaks small, that of the right side moderately 

 excavated to receive that of the left. Interior clear 

 white. Ligament rather large and prominent. Hinge callosity 

 not spoon-shaped, produced ; pallial sinus deeper than wide. 



Length, three fourths of an inch ; height, half an inch ; breadth, 

 three tenths of an inch. 



Inhabits Casco Bay, Maine (^Mighels') ; Massachusetts Bay, on 

 the Middle Bank (^Ai/rcs) ; off Lynn (^Titfts}; east of Scituate ; 

 Casco Bay, off Cheney's Head, Maine {Stimpson) ; off Martha's 

 Vineyard {Ag-assiz) ; off coast of Long Island, thirty-seven fath- 

 oms (Coast Swvci/~) ; Greenland (M'drcli). 



A small species, but its solidity shows it to l)e mature. It is 

 remarkable for its short posterior half. Nor is any mention made 

 of an ossicle ; and the hinge itself hardly allows it to be brought 

 among typical species. Dr. Stimpson has found an ossiculum, very 

 small indeed, in several instances. 



Family MACTRADtE. 



Shell equivalve, usually somewhat gaping at ends ; hinge with 

 an internal cartilage, and sometimes an external ligament also. 

 Animal with the mantle open below ; siphons united and fringed ; 

 gills short. 



Oeniis MACTRA, Lam. 1799. 



Shell elongated, slightly gaping at ends ; beaks prominent ; 

 hinge a prostrate, concave tooth to contain the cartilage, having 



