128 University of Texas Bulletin 



both the ventral and the dorsal margins of the valve and gives to the 

 rostrum an inflated appearance; the notch in the dorsal margin is the 

 deeper and more pronounced one. Posterior to the depressed zone and 

 running slightly divergent to it is a rounded ridge which assumes a con- 

 ical inflation as it approaches the postero-ventral extremity of the ros- 

 trum; on the rostrum dorsal to this ridge is a parallel narrow depression 

 and beyond it a rounded inflation which forms the postero-dorsal extrem- 

 ity of the rostrum. 



The ornamentation consists of thickened, rounded, concentric ribbings, 

 which are broadly flattened on their tops and descend by almost right- 

 angled sides to small plane-bottomed valleys lying between them. On the 

 dorso-ventral axis the widths of ribbings and valleys are about as 2 to 1. 

 The ribs and valleys decrease in width at the anterior margin and at the 

 constriction of the siphonal tube. 



The ventral one-half of the shell contains 12 rounded ribbings ; the dorsal 

 one-half contains 17 rounded ribbings of similar form but of constantly 

 decreasing size, and in addition in the umbonal region 25-30 fainter rib- 

 bings which resemble growth lines. 



These 29 heavier ribbings course posteriorly making broad downward 

 convexities to the region of the siphon tube constriction ; their posterior 

 ends lie in the depressed zone mentioned. They then rise onto the elevated 

 ridge which lies near the anterior end of the rostrum, and course over the 

 inflated rostrum to its dorsal margin. On this elevated ridge the com- 

 ponent growth lines in the ribbings apppear as slight thin imbricated 

 lamellae. Not all of the ribbings run continuously from the anterior to 

 the posterior margin. Towards the anterior margin some of the rib- 

 bings, especially the dorsal ones fuse by twos into broader and more de- 

 pressed flat bands. The ribbings on a whole are more flattened and more 

 consolidated on the anterior margin, and more resolved into growth lines 

 on the posterior margin, in the neighborhood of the siphon-tube. In one 

 ribbing, splitting is seen over the middle half of the shell. In one place a 

 small auxiliary ribbing between two of usual size, is seen over the middle 

 half of the shell. 



The right valve is. somewhat larger than the opening into which the 

 left valve fits, so that the arched, rotund margins of the right valve enclose 

 an elliptical basin shaped visceral space upon which the reduced left valve 

 lies. 



Right valve, interior: Viewed directly from above, the cavity of this 

 valve has an elliptical contour except at the siphonal end where the mar- 

 gins of the ellipse are drawn out so that each margin has an inward facing 

 convexity ; the dorsal margin then slants ventrally and posteriorly to meet 



