Oct. 1847.] 295 



1. Byssoarca lima. 



Trapezoidal, cancellated and granulated ; radii largest on anterior aud pos- 

 terior slopes, but becoming obsolete towards the posterior extremity ; end angu- 

 lar, margin above obliquely truncated ; anterior end widely truncated, nearly 

 direct; basal margin undulated, irregular and very variable in outline ; hinge 

 line crenulated under the beaks, profoundly toothed towards the extremities : 

 cardinal area with lines strongly denned and angulated under the apex. Length 

 2 9-10. Rare. 



2. Byssoarca Misshsippiensis. 



Trapezoidal, with numerous closely-arranged radiating lines, crenulated by 

 fine concentric lines, the crenulation most distinct anteriorly, when the radii 

 are largest; anterior end truncated or a little convex, direct : posterior margin 

 obliquely truncated above ; basal margin widely and profoundly arched ; hinge 

 line long, linear, minutely crenulated, expanded towards the extremities, and 

 with prominent teeth ; cardinal area withfine very closely-arranged lines, angu- 

 lated under the apex. Length 1 6-10. 



Differs from the preceding in having a longer hinge, finer radii, &c, and is a 

 much smaller species and more abundant. 



3. Byssoarca protracta. 



Trapezoidal, elongated, with numerous radiating lines, some of which are 

 double, and' others alternated in size and finely crenulated; dorsal margin, 

 parallel with the base ; anterior margin truncated, posterior a little concave, 

 oblique, end very acutely rounded or subangular ; basal margin slightly con- 

 tracted ; hinge line long, rectlinear, very regular and gradually increasing in 

 width towards the extremities from the apex ; cardinal area wide, depressed 

 concave, with a few fine impressed angular lines. Length 1^. Height i 

 nearly. 



A pretty species of which I found one valve only. 



Avicula argentea. 



Ovate-subquadrangular, thin and fragile, ventricose above, smooth and entire? 

 anterior wing sharply angular ; posterior wing not produced, rectangular at the 

 extremity ; posterior end angular, extending beyond the hinge line and much 

 above the line of the base ; anterior margin and base form a regular rounded 

 outline. Height l. Length 1 4-10. Not common. 



Modiola Mississippiensis. 



Slightly arched, elongated, ventricose, with rather fine clo3ely-arranged radi- 

 ating lines wanting on the anterior side ; region of umbonial slope inflated pos- 

 terior end acutely rounded; beak angulated posteriorly; substance of shell 

 silvery and perlaceous. 2-10th from beak to base. Rare, except in one spot, 

 where I obtained several specimens and fragments. 



Pinna argentea. 



Trianugular, with straight margins and acute summit ; compressed ; substance 

 highly polished and silvery ; valves with longitudinal radii on more than half 



