242 
PALAEONTOLOGY OF NEW YORK. 
ECTENODESMA, Hall. 
Ectenodesma birostratum. 
PLATE XXIII, FIGS. 27-30; AND PLATE LXXXIV, FIG. 20. 
Ectenodesma birostratum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 23, fig-s. 27-30. 
Jan., 1883. 
Shell large; body 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. 
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, 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 which 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, finer 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 striae of 
growth. 
