■ "■' PlIOLAS. 109 



extremity ; the gape is moderate, and both the margins are 

 shghtly convex. The front dorsal edge is incurved, and 

 moderately sloping ; its edge recurved, and spread over the 

 umbonal region, the summit of which is crested by a series 

 of about a dozen deeply-excavated, square-mouthed cells, 

 formed by a dorsal arch supported by perpendicular laminae. 

 The accessory valves are four in number ; the two prin- 

 cipal, which are exactly similar, large, peculiarly fragile, 

 membranaceous, elongated, somewhat wedge-shaped, and 

 almost smooth, or merely marked with the lines of increase, 

 are in fine specimens divided into areas by an oblique pos- 

 terior line, and a more or less shallow or profound anterior 

 excavation, which runs to the extremities of the inner 

 margin (where the valves touch each other), from a slight- 

 ly-projecting point, situated not far from the middle of the 

 opposite margin. Their exact contour varies according to 

 age, the lateral edges being more nearly parallel in the 

 adult ; the anteal extremity, however, is always attenuated 

 to an obtuse point, and the hinder so biangulated that the 

 outer angle is the more prominent ; the interior lateral edge 

 is nearly straight, with a slight retusion near the middle, 

 the exterior one convex and sloping in front, subretuse and 

 straight behind. A solid, transverse, hammer-shaped, tes- 

 taceous plate, most closely attached to their hinder termi- 

 nation, and dove-tailed above into the inner posterior 

 angles, completes the concealment of the cells and umbonal 

 region. This is immediately followed by a narrow and very 

 thin shelly plate, which extends thence more than half-way 

 the distance to the end of the shell. Subumbonal blade is 

 rather large, and moderately wide and elongated, spatulate, 

 curved, flat, broader at its termination, where in expanding 

 it becomes thinner, and internally concave : the hingeplatc 

 is marked with several raised sulci. 



