290 Proceedings of the Royal Physical Society. 



portions of the valve. Beak large, short, and blunt, but not 

 overhanging the hinge line; immediately under it are two 

 large obliquely-placed teeth, which fit into the sockets of the 

 dorsal valve. In the umbonal region of the shell, proceeding 

 from the beak towards the front, is a fine ridge or septum 

 (a groove in casts), with, on each side of it, the dendritic 

 adductor muscular impressions, each bounded on its outer 

 margin by a well-defined groove (ridges in the cast). Im- 

 mediately in front of these adductor impressions is a large, 

 blunt prominence (in the cast becomes a hole or fossa of 

 varying depth, and of a very marked character), with the 

 outer edge much less precipitous than the inner. On each 

 side this prominence are the depressed scars of the cardinal 

 muscles, much deeper and more impressed on their inner or 

 umbonal margins, and gradually dying out laterally towards 

 the sides of the valve ; they are vertically grooved and ridged. 

 (In the cast these impressions become ridged prominences, 

 projecting or scarp-like along their upper edges, and they 

 impinge somewhat on each side over the deep fossa just 

 described.) The interior surface of the valve is pitted and 

 ridged, producing in the cast granules and grooves; the former 

 are continued over the surface of the blunt prominence (fossa 

 in the cast) : the ridges seen on the interior represent the 

 decurrent bases of spines. The exterior surface of the valve 

 is unknown to me, but is described by Mr Etheridge, F.E.S., 

 as " covered with fine, vertical, wavy lines, projecting from 

 which are numerous slender spines." 



The dorsal valve is oval, flat, and very thick, bevelled out- 

 wardly from the interior on the front margin. The latter is 

 apparently continuous, and not indented in any way. Hinge 

 line with rounded alar " angles ; area well marked, but not 

 broad. Cardinal boss thick, strong, and prominent, project- 

 ing from the hinge line at an angle of 119°, with the exterior 

 plane of the valve centrally divided by a groove, which is 

 flanked on each side by a kind of shoulder. Sockets for the 

 reception of the teeth of the ventral valve deep, broad, and 

 so far surrounding the boss as to produce an appearance of 

 isolation in the latter from the remainder of the valve. The 

 cardinal boss is supported on each side by indistinct, oblique 



