146 CARDIADiE. 



one and three fourths of an inch, become solid and strong, and also 

 more tumid, especially behind. Tlie young shells often have the 

 surface variegated with stripes, or angular markings, of a rusty- 

 brown color. This species is also remarkable for gaping posteriorly 

 to a considerable extent. 



Genus CABDITA, Bruguieres. 17S9. 



Shell inequipartite, regular ; hinge with a short, strong, erect 

 tooth under the beaks, and an oblique one stretching along the 

 margin. 



Cardita borealis. 



Fig. 59. 



Shell obliquely sub-cordate, beaks prominent and recurved, with about twenty 

 radiating ribs; margin crcnate; lunule small and deep. 



Cardita borealis, Conrad, Amer. Mar. Conch. 39, pi. 8, fig. 1 (18.31). — Reeve, Conch. 

 Icon. pi. 7, fig. 33. -De Kay, Nat. Hist. New York, 204, pi. 22, fig. 247. — Mid- 

 DEND. Siber. Reise, Moll. ii. 87 ; Beitr. z. Mai. Ross. iii. 29. — Stimpson, Shells of 

 New England, 18. — Mighels, Shells of Maine, 16; Bost. Journ. Nat. Hist. iv. — 

 Gray, Zool. Beechey's Voy. 152, pi. 44, fig. 1. 



Arcturus rudis, Humphrey, MSS. 



Cardita vistita, Deshayes, Proc. Zool. Soc. pi. 17, fig. 10 (1852). 



Shell rounded, obliquely heart-shaped, thick and strong ; inequi- 

 partite ; the beaks elevated and turned forwards, so as almost to be 

 even with the anterior extremity, which is 

 regularly rounded ; posterior margin regular- 

 ly rounded by a much larger curve, which, 

 meeting the base, forms an obtuse angle ; 

 surface raised into about twenty rounded, 

 radiating ribs, which are broader than the 

 grooves between them; these are rendered 

 rough by coarse lines of growth, and covered 

 by a strong, rusty-brown epidermis ; lunule 

 ^' *'"''"^"' very strong, and deeply imprinted, rhom- 



boidal ; ligament small and sunken, nearly concealed ; hinge strong, 

 two teeth in each valve ; in the left valve a small triangular one 

 under the beak, and an oblique, grooved, or partially double one 

 along the posterior margin as long as the ligament ; on the right 

 valve a long, tapering, oblique tooth, fitting beneath the two teeth of 

 the opposite valve, and a more slender one on which the ligament 

 partially rests ; interior white, margin strongly crenate ; impressions 



