LYONSIA. 215 



silvery pearl-like nacre. Both valv^es are decidedly con- 

 vex, or rather subventricose, but tliat which is very slightly 

 the less so, is reflexed at the lower edge (which is pecu- 

 harly sinuated in its outline when viewed from below), 

 and overlapped by the margin of the larger valve. The 

 surface is covered with an extremely thin ash-coloured 

 epidermis, which is much wrinkled concentrically beyond 

 the almost obsolete umbonal ridge, where the valves are 

 compressed, and marked with a shallow excavation. This 

 membrane is frequently covered with grains of sand, and 

 beneath it the shell is closely radiated with delicate raised 

 rugose striae, the interstices of which are microscopically 

 granulated. The dorsal and ventral edges are somewhat 

 parallel ; the front superior margin is very retuse, and in- 

 stead of declining, curves upward at its termination, the 

 hinder dorsal margin is slightly convex, and very mode- 

 rately sloping. The inferior or ventral margin (looked 

 upon from the larger valve) is arcuated, and rises consider- 

 ably behind to meet the slightly retuse and reflected pos- 

 terior edge, which latter almost forms a rectangle with the 

 dorsal margin. The anterior side is regularly rounded at 

 its extremity ; the posterior, which is nearly half as long 

 again, gapes at its narrowed and subtruncated termination. 

 The beaks are obliquely inflected, and are preceded by a 

 rather large lanceolate, and but slightly excavated lunule, 

 which is most easily distinguished by the absence of radi- 

 ating striae. The dorsal edge of the lesser valve somewhat 

 overlaps the other, of which the umbo is decidedly the more 

 prominent. There is no external ligament. The hinge 

 is destitute of fixed teeth, but is provided with a movable, 

 somewhat heart-shaped, testaceous ossicle, which is united 

 to both the valves by a cartilaginous deposit. 



The animal of this interesting bivalve has received consi- 



