12 



convex shell, the beak not so far back, and much more obtuse- 

 ly rounded before. The few specimens in the cabinet of my 

 friend Lesueur are not tinged with purle within. 



It has some resembiance to Lister's fig. 9, of pi. 154, which 

 Dillwyn calls fucatus, but the present is a longer shell, with 

 somewhat more elevated beaks, more rounded at the base,&c. 



A. grandis. Slidl very large, subovate: disk unequally 

 wrinkled and undulated transversely, dark yellowish brown: 

 umbo elevated: beak slightly elevated, with, generally, two or 

 three small sinuous, acute undulations: hinge margin slightly 

 arquated, sometimes nearly rectilinear, somewhat angulated 

 at its anterior termination, thence the edge descends by a 

 nearly rectilinear, or slightly concave line to the anterior mar- 

 gin, which is considerably narrowed: sinus of the hinge mar- 

 gin, concave: posterior margin widely rounded: within white, 

 margined, particularly before, with dusky. 



Length, nearly five inches; breadth, over seven and a half 

 inches. Convexity, three and a quarter inches. Inhabits 

 Fox River of the Wabash. 



The great size of this species, distinguishes it from any other 

 1 have met with. A favorable specimen at first view, corres- 

 ponds with the figures on pi. 205 of the Encycl. Mcth., named 

 by Lamarck, A. trapezialis ; but the differential character is, 

 in the words of Lamarck, " La sinus de la lame cardinale est 

 grandc, ct form un angle rentrant, aigu." He described from 

 a speci?nen in his own collection, and as the figures exhibit 

 this character, there can be no doubt that our shell is a distinct 

 species. 



The finest specimen I have seen was presented to me by 

 Mr. Oliver Evans, who obtained it in Fox River, which is a 

 serpentine arm of the Wabash. 



A. imptird. Subovate, fragile, a little compressed, dull yel- 

 lowish brown: umbo much eroded, exhibiting a chalky white 

 stratum, then a dull wax yellow surface: beak a little undulat- 

 ed, hardly elevated above the general curvature: hinge margin 

 forming a large angle with the base, being anteriorly elevate/ 

 into an obtuse angle, wdience the edge descends almost recti 

 linearly to the anterior margin, which is equally rounded with 

 the posterior margin. 



Length, one inch and three-tenths; breadth, less than two 

 inches. Convexity, less than seven-tenths. Inhabits Mexico. 



