132 University of Texas Bulletin 



of the visceral space and proceeds backward to a point underneath the 

 posterior adductor muscle scar. Here it turns backward making a 45 

 degree angle, then turns straight dorsally, making with its former course 

 a 90 degree angle, and reaches the lower angle of the scar. This rounded 

 triangular scar likewise lies on an elevation formed by the inner contour 

 mentioned, which passes dorsally and anteriorly, then turns anteriorly 

 reaching the ventral border of the tooth. It thus leaves dorsal to it and 

 posterior to the tooth, a kidney-shaped pit, into whose posterior border a 

 small tooth projects. 



Left valve, exterior: This valve is smaller and distinctly shorter than 

 the right valve, but has almost the same depth. Its smaller length is due 

 to the shortness of the rostrum which fails to cover about one-half the 

 rostral area of the right valve, and thus leave a narrow wedge-shaped gap 

 for the siphon tubes. The left valve is in contour an obliquely truncated 

 ellipse. Seen from above, it has an umbonal angle of about 100 degrees 

 and an almost straight antero-dorsal margin. This continues as a sharply 

 and evenly rounded curve into the ventral margin which is gently curved, 

 but which posteriorly curves more sharply, reaching the tip of the ros- 

 trum. The postero-dorsal margin is roughly a straight line, except for a 

 slight bulging near the posterior half. The valve has a marginal gen- 

 iculation of about two-thirds the height of the corresponding one on the 

 right valve, and consequently a basin-like form for the entire valve. 



The ornamentations consist of growth lines and weak concentric ribs, 

 of which there are about seven below the geniculation, and 26 or more 

 above it, the ribs decreasing in size in the dorsal one-third of the shell. 



The ventral one-half of the valve, above the geniculation, bears about 

 12 ribs. 



Left valve, interior: This valve has a truncate elliptical margin. It 

 may be divided into two regions : the visceral cavity and the rostrum. The 

 anterior part lacks the prominent shelf seen in the other valve. The 

 visceral cavity is a subquadrate depressed area from which a shallow 

 curved recess extends under the beak. Its margin is separated from the 

 posterior margin of the valve by the triangular shaped rostrum whose 

 inner surface is practically plane ; the dorsal and narrow two-thirds of the 

 triangle is occupied by the elliptical scar of the posterior adductor muscle ; 

 the anterior edge of this scar is smooth and forms the rim of the "shelf" ; 

 from it a perpendicular wall descends to the floor of the visceral cavity. 

 The ventral and wider one-third of the rostrum is the space for the siphon 

 tubes, and is plane with the floor of the visceral space. 



From the lower end of the posterior adductor scar the narrow ribbon- 

 like pallial line descends ventrally for a short distance, turns sharply pos- 



