157 



unequal teeth, one short, straight, situated under the beaks; the other ob- 

 lique, marginal, and extending under the corselets. 



C. sulcata, C. bicolor, C. subaspera, C. citrina, 



' ajar, ' depressa, ' nodulosa, ' sublaevigata, 



' turgida, ' phrenetica, ' concamerata, ' corbularis, 



' squamosa, ' crassicosta, ' sinuata, ' lithophagella, 



' intermedia, ' rufescens, ' aviculina, 4 fossil species. 



' trapezia, ' calyculata, 



Cypricardia. Shell free, equivalve, inequilateral, obliquely or trans- 

 versely elongated. Three cardinal teeth under the beaks, and one lateral 

 tooth extending under the corselet. 



C. Guinaica, C. rostrata, C. coralliophaga, 3 fossil species. 



' angulata, 



Hiatella. Shell equivalve, very inequilateral, transverse, gaping at the 

 superior margin, hinge with a small tooth in the right valve, and two oblique 

 teeth, larger in the left valve. Ligament external. 



H. arctica. 



Isocardia. Shell equivalve, heart-shaped, ventricose, the beaks distant, 

 diverging, spirally turned on one side, two flat, entering, cardinal teeth, one 

 of them curved, and sunk under the beak; one elongated, lateral tooth, situ- 

 ated under the corselet. Ligament external, forked on one side. 



I. cor, I. Moltkiana, I. semisulcata, 1 fossil species. 



ARCACEA.— Four Genera. 



Cardinal teeth small, numerous, entering, and disposed in each valve, in a 

 straight, arched or broken line. 



CucuLLiEA. Shell equivalve, inequilateral trapeziform, ventricose; the 

 beaks distant, separated by the facet of the ligament; the anterior muscular 

 impression forming a projecting angular, or auricular margin; hinge linear, 

 straight, with small transverse teeth, and having, at the extremities, from 

 two to five ribs parallel to it. Ligament wholly external. 



C. auriculifera, 1 fossil species. 



Arca. Shell transverse, sub-equivalve, inequilateral; the beaks distant, 

 separated by the facet of the linament; hinge in a straight line, without limbs 



