P8AMM0BIA. 275 



less twisted, and not perfectly equal in depth. When 

 fresh taken it is usually covered with a dull ashy olive- 

 coloured epidermis, beneath which the surface, which pos- 

 sesses but little if any lustre, is either rayed with madder- 

 lake and white, (the preponderance of colour varying with 

 the individual) or marbled in a linear fashion, with the 

 former, on a whitish or pale-coloured ground. It is closely 

 striated for its entire length with raised concentric lines, 

 which not unfrequently at their posterior termination dentate 

 the hinder dorsal edge. These strise are decussated beyond 

 the umbonal ridge, which is indicated by a sharp elevated 

 line, by a few fine radiating ones, which in the adult, how- 

 ever, do not quite proceed to the extremity of the shell. 

 The ventral margin is nearly straight in the middle, but 

 ascends (and convexly so anteriorly) at either extremity. 

 The dorsal edges are flattened above ; the front one is 

 slightly convex, and but little sloping, the hinder one is 

 subretuse, and scarcely at all declining. The front extre- 

 mity of the shell is rounded and a little tapering, the 

 hinder termination is perfectly biangulated, the lower angle 

 projecting further than the upper. There is a slight ven- 

 tral flexure before the umbonal ridge ; the ligament is long, 

 prominent, moderately large, rich brown in the adult, and 

 brownish-yellow in the young ; the beaks are acute but 

 small, and not much projecting. The interior, which is 

 highly polished, is white or lilac ; the teeth, which are 

 rather small, consist in the left valve of a deeply cloven 

 narrow central, and a very small rudimentary laminar one, 

 the latter lying obliquely behind the former, and being- 

 very caducous is usually wanting ; in the right valve, of a 

 simple anterior, and a bifid posterior one, the former of 

 which is the more solid. 



Individuals which have attained to two inches in length, 



