324 [December, 1S43. 



Arca scalaris. Obliquely rhomboidal, elevated, ventricose, ribs about twen- 

 ty-three, broad, square, prominent, profoundly and robustly crenate, wider than 

 the interstices, seven on the posterior slope, prominent; posterior slope flattened ; 

 umbonial slope angulated ; summit elevated, narrowed ; anterior margin oblique- 

 ly truncated ; anterior basal margin obliquely subtruncated ; posterior extremity 

 subangulated ; beaks remote; area with transverse slightly impressed lines; 

 cardinal teeth irregular, oblique towards the extremities of the hinge line ; within 

 with furrows corresponding to the ribs ; margin profoundly crenate. Length, 

 two inches ; height, one and a half inches. 



Locality. Petersburg, Virginia. Mr. Tuomey. 



Allied to A. incongrua, Say. The description applies to the left valve only, 

 as the opposite one has not yet been found. 



Ctrena. 



Ctrena densata. Subtriangular, thick, convex, with robust lines of growth ; 

 anterior margin obtusely rounded ; basal margin profoundly and regularly curved 

 to the posterior extremity, which is subtruncated, direct, and greatly above the 

 line of the base ; beaks central, summits elevated ; teeth large, robust, very pro- 

 minent, three in one valve, and two in the opposite ; middle tooth of the right 

 valve bifid ; lateral teeth elongated, robust, anterior tooth truncated, suddenly 

 deflected at the extremity ; posterior tooth distant. Length, one and a half 

 inches ; height, one inch and seven-eighths. 



Locality. Vicinity of Petersburg, Virginia. Mr. Tuomey. Rare. 



Mactra. 



Mactra triquetra. Triangular, thick, ventricose ; anterior margin obtusely 

 rounded; posterior margin obliquely truncated ; extremity angular; anterior 

 and basal margins regularly curved : posterior basal margin obliquely truncated, 

 and the disk slightly flattened above ; umbonial slope forming a right angle 

 with the posterior depression ; beaks slightly remote, central, summits profound- 

 ly elevated ; lateral teeth robust; fosset small, ovate. Length, one and a quar- 

 ter inches ; height one inch. 



Locality. Vicinity of Petersburg, Virginia. 



I found a single perfect valve of this species, which is much larger and pro- 

 portionally shorter than M. congesta, and differs especially in the beaks being 

 distant from the hinge margin. 



Venus. 



1. Venus capax. Cordate, suborbicular, ventricose, with concentric lamel- 

 liform prominent lines ; posterior margin curved, extremity truncated, direct, 

 and remote from the line of the base ; basal margin profoundly curved ; lunule 

 dilated, cordate, defined by a groove, and not distinctly impressed ; inner margin 

 finely crenulated. 



Locality. Pamunkey river, Kent county, Virginia. Mr. Tuomey. 

 This shell is of a more rotund, tumid form than any of the species allied to 

 V. mtrcmaria, and much more capacious ; the lunule is shorter and wider. 



2. Venus permagna. Subtriangular, cordate, profoundly ventricose, thick 

 and ponderous, with coarse concentric recurved rib-like lines, elevated, closely 



