No. 109.] 137 



Ortiiis perversa (n. s.). 



Shell iiiisymmetrically subelliptical in outline, much wider than 

 long; valves unequally convex ; area large and confined to the 

 ventral valve, extending to the hinge extremities ; hinge-line 

 shorter than the greatest width of the shell : dorsal valve most 

 convex near the umbo or above the centre of the shell, and 

 sloping somewhat abruptly to the hinge -line; extremities 

 rounded : ventral valve most convex at the beak, which is 

 slightly arcuate and often distorted, depressed below ; area 

 nearly vertical; foramen closed. Surhice marked by distant 

 elevated strlee, which bifurcate several times before reaching the 

 margin, as well as increase by interstitial addition ; concentri- 

 cally marked by closely arranged undulating lines of growth. 



This shell is of the type of Orthix umbracuhim, and presents the ir- 

 regularities of form common on that shell. Among the specimens examined, 

 there are no symmetrical forms ; both valves showing some slight degree 

 of distortion, which appears to have been the normal condition of the shell. 

 The species is readily distinguished among other species of the Hamilton 

 group, even by fragments of the shell from the unsymmetrical form, the 

 distant radiating stria;, and the concentric stride, which are closely arranged, 

 abruptly undulating between the strong radiating striae. 



Geological position and locality. In the shales of the Hamilton group, 

 Erie county, N.Y. 



Strophomexa (Strophodonta) demissa. 



Strophomena demissa, Conrad : Jour. Acad. Nat. Sci. Phil. 1842, Yol.viii, 



pa. 258, pi. 14, f. 14. 



Shell semielleptical, usually wider than high (length and breadth 

 often nearly equal) ; hinge-line equal to, greater or less than the 

 width of the shell, abruptly contracted below the hinge extre- 

 mities which are often auriculate : ventral valve very regularly 

 convex, sometimes gibbous, greatest elevation nearly central : 

 dorsal valve moderately concave, rarely following the convexity 

 of the opposite valve; area conspicuous, wide, sublincar, that 

 of the ventral valve arcuate, that of the dorsal valve convex 



