242 PALAEONTOLOGY OF NEW YORK. 



ECTENODESMA, Hall. 



ECTENODES.MA BIROSTRATUM. 

 I'LATIi XXUi, Flo.-.. -.'T-iu; ami PLATK LXXXIV, KIG. M 



Ecleiiodesma birnslratuiii, H.vi.i,. Pal. N. V., vol. v, pt. 1. Plates and Explanations; PI. 23, figa. 27-30. 



Jan., 1883. 



Shell large ; body ovatu, ()bli(|U(j ; lieiglit greater than the length ; margin 

 regularly curving from the base of the anterior wing to the post-basal 

 margin, where it is somewhat abruptly recurved. 



Left valve regularly convex below, gibbous in the umbonal region, some- 

 what arcuate, the point of greatest convexity being about the middle of its 

 length. Right valve concave below, depressed-convex in the middle, convex 

 on the umbo. 



Hinge-line straight, iinicli longer than the length of the shell, and, in 

 extreme specimens, more than once ami a half greater than the length of 

 the shell. 



Beaks acute, anterior to the middle of the shell, inclined forward, and 

 arching over the hinge-line. Umbonal region gibbous, limited on the 

 anterior side by a shallow undefined sulcus, and on the posterior side by the 

 abrupt depression ol' the body, subtending an acute angle. 



Anterior wing large, triangular ; margin concave ; extremity produced to 

 an acuminate extension. Byssal sinus shallow and undefined. Posterior wing 

 large, triangular, joining the body below the middle of its height; margin 

 concave ; extremity produced into an acute termination. 



Test of left valve marked with regular and even radii which are rounded 

 above, fattened and sometimes bifurcate below ; similar but more acute radii 

 continue over the posterior wing. In the right valve the ra\ s are more 

 numerous, hner and sharp, and are continued upon the posterior wing with a 

 little less force than on the body of the shell ; and very much subdued upon 

 the anterior wing; entire surface marked In' fine, even, concentric striae of 

 growth. 



