Lamellibranchiata. 399 



ECTENODESMA, Hall. 



ECTENODESMA BIROSTRATUM. 



Ectenodesma Urostratum, Hall. Pal. 1ST. Y., vol. v, pt. 1. Plates and Ex- 

 planations : PI. 23, figs. 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 mid- 

 dle of its length. Eight 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 un- 

 defined. 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. The 

 specimens show an oblique lateral tooth, with obscure indications of 

 anterior teeth or folds. Ligamental area narrow, marked with fine 

 parallel striae. 



A left valve has a length of 45 mm., height 50 mm., hinge-line, 

 when entire, about 75 mm. A small right valve has a length of 

 34 mm., height 27 mm., hinge-line about 45 mm. 



This species is distinguished by the great and nearly equal extent 

 of the hinge-line on both sides of the beak ; by the peculiar form of 

 the body of the shell ; and by its surface characters, which are unlike 

 any yet observed. 



