440 Systematic Paleontology 



Description. — " Shell of medium size, elongate, with strongly convex 

 smooth valves. Pedicle valve arcuate from beak to base, greatest con- 

 vexity in the umbonal region. Beak curved to a 90° angle with the edge 

 of the valve apparently truncated by a circular foramen. The center of 

 the valve is marked by a median depression which begins a short distance 

 below the beak and extending forward gradually broadens without much 

 deepening. Near the anterior end this sinus is gently rounded, sometimes 

 almost flat bottomed. On the anterior end the sinus is marked by a slight 

 rounded projection. Surface marked by concentric striae which at in- 

 tervals in some specimens become strong wrinkles near the front. 



" On the interior the deltidial margins are supported by delicate dental 

 septse. These arise from the bottom of the valve on either side of the 

 center and at first are inclined outwards for about one-half their height 

 and then turn rather abruptly upwards making a marked angle. Near the 

 upper end they bend outward again to the margins of the delthyrium. 



" At the lower angle a thin short plate springs outward and upward con- 

 necting the septum with the shell. This plate is marked in the internal 

 mold by a pronounced slit, cutting the mold of the lateral rostral cham- 

 bers. The aspect of the whole is that of a broad spondylium resting on 

 the bottom of the valve and supported laterally by the secondary lamella. 

 Anteriorly the dental lamellse extend as low, slightly outward curving 

 ridges, which between them enclose a longitudinally striated muscular 

 area. 



" Brachial valve subquadrangular, the width slightly greater than the 

 height ; somewhat less convex than the pedicle valve, and regularly arched 

 without median fold. In some ca?es the faintest longitudinal depressions 

 occur near the front, in the lateral third of the slope, thus giving a sug- 

 gestion of a median fold. The beak projects slightly above the cardinal 

 line, being incurved. The postero-lateral margins are more or less regular 

 curves, the antero-lateral ones have their outline curved to a larger radius 

 thus making the sinus appear rather truncate. The anterior margin is 

 slightly emarginate corresponding to the projection of the pedicle valve. 

 Surface marked by lines of growth and in some specimens by irregular 

 wrinkles. On the interior a strong sharp septum extends from the beak 



