MARINE MOLLUSCA OF THE UNITED STATES. 135 



cate behind. Beaks prominent, with an obtuse cavination extend- 

 ing to the angle of the posterior and basal margins; beak of the 

 right valve perforated to receive tlie point of the left. Right valve 

 more convex and extending somewhat beyond the left, and both 

 slightly gaping. Hinge toothless, but with rounded eminences. 

 Epidermis thin, with concentric undulated strise. Pallial impres- 

 sion with an acute angular sinus. No ossiculum. 



New England, northwards. 



2. T. MYOPSis, Beck, Fig. 303. 



Moller, Index Moll. Groeul. 18. 1842. 

 T. Coutliouyi, Stimpson, Bost. Proc, iv. 8. 1851. 



Shell small, white, orbicular-ovate, compressed ; beaks nearly 

 median, narrowed and rounded in front, more pointed and trun- 

 cate behind, gaping ; surface with rather elevated concentric lines ; 

 hinge callus thickened backwards, without any distinct spoon- 

 cavity. Ossiculum very minute. 

 Height 17.5, length 25 mill. 



New England, northwards. 



Mr. Jeffreys considers this species a synonym of T. triaicafa, 

 Brown (Brit. Conch. 1827), but I think that Brown's figure rep- 

 resents a very different shell. In thus adopting Brown's name 

 for this species, the following species, T. truncata, M. and A., has 

 its name preoccupied, and Mr. Jeffreys proposes to change it to 

 T. septentrionalis (Ann. and Mag. Nat. Hist. 238, Oct. 1872). I 

 do not adopt the new name because I am not satisfied that 

 Brown's species is really distinct from T. distorta, Montagu. 



3. T. truncata, Mighels and Adams. Fig. 304. 



Bost. Journ. Nat. Hist., iv. 38, t. 4, f. 1. 1843. 



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

 beaks at posterior fourth, posterior margin broadly truncate. 

 Beaks small, the right one excavated to receive that of the left 

 valve. Epidermis yellowish, within white. Ligament rather 

 large and prominent. Hinge callosity not spoon-shaped, produced. 



Heiglit 12.5, length 19 mill. 



New England, northwards. 



See remarks under preceding species. 



Genus PANDORA, Bruguiere. 

 Encyc. Meth. t. 250. 1792. 



Animal with mantle closed, except a small opening for the 

 narrow, tongue-shaped foot ; siphons very short, united nearly 



