456 PROCEEDINGS OF THE ACADEMY OP 



margin convex along the midlde, and rather distinctly concave in outline near 

 the hinge above ; base very abruptly rounded, or subangular near the front ; 

 anterior side oblique, nearly straight, broadly and distinctly flattened at right 

 angles to the plane of the valves; umbonal slopes extremely prominent, and 

 distinctly angular, ranging at an angle of 58 with the hinge line ; beaks ter- 

 minal, elevated a little above the hinge, incurved, acutely angular, and 

 strongly compressed at right angles to the hinge. Surface of casts retaining 

 rather obscure marks of growth. 



Height, measuring at right angles to the hinge, 3 - 32 inches ; length parallel 

 to the hinge, 3 inches ; length, measuring from the beaks obliquely, to the 

 most prominent part of the base, 3 '35 inches ; convexity of the valves, 1'30 

 inch. 



Locality and position. Chester, Illinois. Chester Limestone of the Lower 

 Carboniferous series. 



Myalina concentrica. Shell small, thin, rather compressed, subquadrate, 

 a little higher than long, not very oblique ; hinge straight, slightly less than 

 the length of the valves below ; anterior side a little arcuate, ranging at an 

 angle of about 85 Q with the hinge, and curving rather abruptly inward from the 

 umbonal slopes ; posterior side compressed, straight, or very slightly sinuous 

 in outline above, and intersecting the hinge at about the same angle as the 

 anterior border ; base regularly rounded ; beaks pointed, incurved, and 

 scarcely rising above the hinge ; umbonal slopes rather compressed ; surface 

 marked by slender, somewhat obscure, regularly arranged lines, separated 

 by smooth spaces some eight to ten times their own breadth. 



Height, - 67 inch ; length, 0*59 inch; convexity of a left valve, about 0-22 

 inch. 



Locality and position. Spergen Hill, Indiana ; from an Oolitic bed contain- 

 ing great numbers of small fossils, described by Prof. Hall, and placed by him 

 on a parallel with the Warsaw Limestone of the Lower Carboniferous series. 



Mtalina eecukvieostra. Shell of medium size, rather thick, obliquely 

 trigonal, ovate, inequivalve, (the left valve being more convex, and thicker 

 than the other, ) very gibbous, and narrowly rounded along tbe umbonal slopes ; 

 hinge nearly or quite straight, more than equalling the height in young 

 specimens, but proportionally a little shorter in mature shells ; posterior 

 margin a little convex, ranging nearly at rigbt angles to the hinge above, and 

 rounding gradually forward below ; base narrowly rounded ; anterior margin 

 oblique, or ranging at an angle of about 55 p with the hinge, slightly convex 

 in outline, and separated by a shallow, oblique depression, from tbe umbonal 

 ridge above, and broadly sinuous along the middle ; beaks small, pointed, ter- 

 minal, that of the left valve being spirally incurved, so as to make nearly one 

 entire turn at the extreme point, which is directed obliquely forward ; surface 

 marked by distinct lines, and imbricating lamina? of growth, which are quite 

 prominent on the anterior side, and near the cardinal margin of the left valve, 

 but more obscure on all parts of the other. 



The ligament area is comparatively narrow, and marked by longi- 

 tudinal furrows ; immediately under the beak the anterior margin is thickened , 

 so as to present a broader area than the hinge, from which it is separated, in 

 the left valve, by an oblique groove, and in the other by a prominent cor- 

 responding ridge. The posterior muscular impression is large, narrow, ovate, 

 acutely angular above, rounded below, and ranging nearly parallel to the pos- 

 terior border. The pallial line is well defined ; anterior muscular scar small, 

 oval, and located near the beaks. 



Length, measuring forward and upward from the postero-basal extremity 

 to the beaks, 2-10 inches ; height, 1-67 inch ; convexity, about 1 inch ; length 

 of posterior muscular scar, 0*75 inch ; breadth of do. at lower extremity, 0*28 

 inch. 



Locality and position. Near Springfield, Illinois. Coal Measures. 



[Oct. 



