COMPOSITA 487 



to or greater than the length, the greatest width near or anterior to the 

 mid-length of the shell, the fold and sinus usually well developed anter- 

 iorly. The dimensions of three examples from the Salem limestone are : 

 length 8.9 mm., 13.8 mm. and 18.5 mm., width 8 mm., 12.7 mm., and 19.5 

 mm., thickness 6 mm., 9 mm. and 10.8 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly from the umbonal region to the cardinal margin, and also in 

 the narrower examples to the lateral margins, the curvature from the 

 beak to the front margin becoming regularly less convex anteriorly ; the 

 mesial sinus originating at or near the beak as a narrow, flattened or 

 slightly depressed line, anteriorly it becomes stronger, in the narrower 

 examples it is rounded in the bottom, in the wider forms it is broader 

 and often with a more or less distinct groove-like furrow along the 

 median line, the lateral margins of the sinus are ill-defined, but in some 

 examples, especially the broader ones, there is a more or less well-defined 

 furrow upon each lateral slope of the valve, a little outside of the margin 

 of the sinus and parallel to it, these furrows sometimes originating in the 

 umbonal region and becoming stronger towards the anterior margin; 

 the beak more or less strongly incurved, truncated obliquely to the plane 

 of the valve, and perforated by a large subcircular foramen; cardinal 

 area obsolete, the delthyrium broadly triangular, connected at its apex 

 with the foramen, nearly or quite filled by the beak of the opposite valve. 



Brachial valve somewhat trilobate, about equally convex with the 

 pedicle, the greatest convexity usually near the middle ; mesial fold 

 usually originating in the umbonal region, only weakly developed in the 

 posterior half of the shell, becoming more prominent in front, in the 

 narrower forms it is rounded on top, but in the broader individuals it is 

 flatter and is sometimes marked by a narrow, shallow, median furrow 

 which originates near the middle of the shell and continues to the front 

 margin, on each side of the fold in the more strongly trilobed forms 

 there is a rounded sinus, originating near the middle of the valve and 

 increasing in strength towards the front, the lateral slopes curving more 

 or less abruptly to the lateral margins; beak strongly incurved beneath 

 that of the opposite valve and nearly or quite filling the delthyrium. 



The surface of both valves is marked by more or less regular con- 

 centric lines of growth which are sometimes crowded toward the front, 

 otherwise the surface of the shell is smooth. 



Remarks. This species is an exceedingly variable one, and extreme 

 forms, if found separate and isolated, would be considered without hesi- 

 tation as distinct species. The variation in size is considerable, the 

 smaller specimens from the diminutive Spergen Hill fauna sometimes 

 being less than 5 mm. in length, although apparently adult individuals. 

 The larger examples are often as much as 20 mm. in length. The propor- 



