242 PAL.EOSTOLOGY OF NEW YORK. 



ECTENODESMA, Hall. 



ECTENODESMA BIROSTRATUM. 



1-1. ATI: Will. FIGS. 37 10; UOJ IM.AT1-: J. XXXIV. FIG. 20 



todeama birostratum, IIm.i.. Pal. N. Y.. vol. v, pt. 1. Plates and Explanations: PI. 23, figs. 27 30. 



Jan., L883 



Shell large; bodj ovate oblique; height 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. 



Lett 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, much longer than the length of the shell, and, in 

 extreme specimens, more than once and 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 of 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 whicb are rounded 

 above, flattened and sometimes bifurcate below: similar but more acute radii 

 continue over the posterior wing. In the right valve the rays are more 

 numerous, liner 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 by fine, even, concentric' stria' of 

 grov tb. 



