72 MACTRAD^. 



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 Enylaml, 22 (18.51); Mar. Invert. Gr. Manan, 21 

 (1853) ; Check Lists, Smith. Inst. 3. — Morch, Gi-iJnl. BliJddyr, 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 

 Fig. 386. venti-al angle ; anterior dorsal margin nearly hori- 



zontal ; surface with numerous and conspicuous lines 

 of growth, and covered by a ])ale yellowish epider- 

 T. truncata. uiis I 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 (^Ayres^ ; off Lynn ( Tufts) ; east of Scituate ; 

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

 Vineyard (^Agassiz) ; off coast of Long Island, thirty-seven fath- 

 oms {Coast Survet/) ; Greenland (3I6rch). 



A small species, but its solidity shows it to be mature. It is 

 remarkal)lc for its short posterior half. Nor is any mention made 

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

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

 small indeed, in several instances. 



Family MACTEAD^E. 



Shell cquivalve, 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. 



OenilS ITIACTRA, Lam. 1799. 



Shell elongated, slightly gaping at ends ; beaks prominent ; 

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



