UNITED STATES AND ARCTIC AMERICA 191 



slightly prominent; epidermis ochraceous or yellowish brown, obliquely radiate; 

 cardinal teeth thick, striate, double in both valves ; lateral very long, strong, corru- 

 gate, nearly straight and lamellar ; nacre white, purple or salmon color and very 

 iridescent. 



Prop. Acad. Nat. Sci., 1861, p. 302. 



Hob. — Connecticut River, at Northampton. At Springfield, by S. Shurtleff, M. D. 

 Below Hartford, T. R. Ingalls, M. D. Neuse River, N. C, E. Emmons, M. D. 



My cabinet and cabinets of Dr. Shurtleff, Dr. Ingalls and Dr. Emmons. 

 Diam. 1, Length 2-2, Breadth 4-4 inches. 



Shell smooth, oblong, very much compressed, flattened at the sides, obtusely 

 biangular behind, obliquely rounded before and very inequilateral ; substance of 

 shell rather thick, thicker before ; beaks a little prominent : ligament long; thick and 

 dark brown ; epidermis ochraceous or yellowish brown, with oblique rays, dark on 

 the posterior slope, with numerous rather close marks of growth ; umbonial slope 

 very slightly raised ; posterior slope very narrow, long elliptical, raised into a 

 carina, very slightly sulcate, with dark brown or greenish broad rays ; cardinal 

 teeth thick, very much striate, perpendicular to the base and double in both valves; 

 lateral teeth very long, stout, roughened, nearly straight and lamellar; anterior 

 cicatrices distinct, large and corrugate ; posterior cicatrices confluent, very large and 

 well impressed ; dorsal cicatrices in a row from the base of the cardinal tooth along 

 the plate over the centre of the cavity of the beaks ; cavity of the shell very shallow 

 and wide ; cavity of the beaks very shallow and rounded ; nacre white, purplish or 

 salmon color and very iridescent. 



Remarks. — I have a number of specimens of this species from the habitats men- 

 tioned above. The first specimen was found by myself some thirty }*ears since in 

 the Connecticut River, at Northampton, Massachusetts. Subsequently, in 1854, at 

 the same place, I found two other smaller individuals, inhabiting with complanatus. 

 from which species I had before feared to separate it. Subsequently, I received speci- 

 mens from several friends coming from different habitats. That so far removed as the 

 Neuse River near Raleigh took me rather by surprise. It certainly belongs to that group 

 of which complanatus may be considered the type. It may easily be distinguished by 

 its great flatness, its biangular posterior margin and its thickening on the anterior half 

 of the valve. Although I have an excellent suite of old and young, none are perfect 

 enough at the tips of the beaks to reveal the character of its undulations, which 

 will, probably, be found like those on complanatus. Some of the specimens have 

 scarcely any rays on the sides, while others have numerous broad or narrow green- 

 ish rays over the whole disk. On the posterior slope the rays are close and give a 

 lark hue to the whole of the portion of the slope. The biangular form of the 



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