Oct. 1847.] 291 



very oblique, nearly straight, extremity acutely rounded, much above the line 

 of the base, which is regularly curved; summit prominent; lunule elongated, 

 defined by an impressed line; umbonial slope nearly terminal, angulated and 

 carinated by a thin line. Length 1 8-10. Height 1 4-10. 



Proportionally more elevated than M. prcstenuis, of Claiborne, Alabama, and 

 a much larger species. Rare. 



2. Mactra fimcrata.. 



Triangular, small, convex; much longer than high, equilateral; posterior 

 end angular. Length 3-10. Rare. 



A mphidesma Mississippiensis. 



Oblong-oval, somewhat compressed, inequilateral, smooth, with a few distant 

 concentric impressed lines; posterior side with a slight fold, end obtusely 

 rounded ; anterior end rounded ; cartilage pit very narrow, elliptical ; lateral 

 teeth in the right valve distinct, in the left wanting? Length 1 1-10, 

 Height 7-10. 



1, Psammobia papyria. 



Oblong-oval, or somewhat rhomboidal ; very thin, compressed ; posterior side 

 rather wider than anterior; and posterior to the umbonial slope, which is unde- 

 fined, there are concentric lamellaeform lines; anterior margin obliquely 

 rounded ; dorsal margin parallel with the base. Length U. Rare. 



2. Psammobia lintea. 

 Oblong, compressed, with rather fine, very regular concentric lines closely 

 arranged ; anterior margin acutely rounded, the extremity in a line above the 

 middle of the valve; posterior side shortest; hinge line rectilinear, oblique ; 

 posterior side with acute lines larger and more prominent than those of, the 

 middle and anterior side; umbonial slope subangular. Length 1J. Rare. 



Crassatella Mississippiensis. 



Ovate-trigonal, inequilateral, thick and ponderous ; surface coarsely striated ; 

 summits flattened, sulcated; umbo plano-convex, with numerous sulci, obsolete 

 behind the umbonial slope, which is angulated and subcarinated above; pos- 

 terior extremity truncated, direct; basal margin slightly contracted anterior to 

 the umbonial slope; inner margin crenulated. Length 3. Common. 



This species is variable in outline, and allied to C. tumida of the Paris 

 basin. 



1. Car Hum eversum. 



Ovate, elevated, subequilateral; ventricose, thin, with numerous approximate, 

 slender, rounded ribs distinctly crenulated anteriorly towards the margins; um- 

 bonial slope subangulated; posterior extremity subtruncated, direct; summit 

 very prominent; ribs crenulated on the sides, in the middle of the valves, or 

 finely aculeated. Height 1 2-10. Length not quite as much. Rare. 



2. Cardium glebosum. 



Ovate, ventricose, with numerous flat ribs, slightly carinated on the margins 

 and numerous approximate, prominent arched scales. Height l\. 



Rare, and obtained only in fragments. 



