94 INVERTEBRATA OF MASSACHUSETTS. 



Cardi'ta borea'lis. 



Shell obliquely sub-cordate, beaks prominent and recurved, tcith 

 about twenty radiating ribs ; margin crenate ; lunule small and deep. 



Figure 59. 



State Coll., No. 189. Soc. Cab., No. 1930. 



Cardlta borealis, Conrad; Jlmer. Mar. Condi., 39, pi. 8, f. 1. .Sowekby; JIpp. 



to Beechey's Voy., pi. 44, f. 1. 

 Arclurus rudis, Humphrey ; Mss. 



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

 equilateral ; the beaks elevated and turned forwards, so as almost 

 to be even with the anterior extremity, which is regularly round- 

 ed ; posterior margin regularly 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, rhomboidal ; 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, taper- 

 ing, oblique tooth, fitting between the two teeth of the opposite 

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

 rests ; interior white, margin strongly crenate ; impressions dis- 

 tinct. Length 1 inch, height 1 inch, width y\ inch. 



Found along the whole coast of Massachusetts, and is one of 

 the most common shells found in fishes. It is a more northern 

 shell, and is found along the coast of Maine, and in the Arctic 

 seas, of a very large size. 



The shape of the shell is much varied by age. In the young the 

 beaks are nearly central, very little elevated, and scarcely recurved ; 

 but the posterior portion advancing in growth faster than the anterior, 

 produces the obliquity of the old shell. It is closely allied to C. tri- 

 dentdta, Say, but it grows to a much larger size, is more inequilateral, 

 and has two teeth in the right valve, while that shell has but one. 



