PISIDIUM. 121 



valves, is rounded oval, and but slightly oblique. It is 

 tumid, the profundity nearly equalling the breadth, glossy, 

 and clothed with an epidermis of a greenish black or dusky 

 ochraceous hue, margined by a zone of a dirty yellowish or 

 somewhat orange-coloured tint, which in the young is more 

 especially broad. Occasionally, but rarely, the entire sur- 

 face, which is finely striated in a concentric direction, is of 

 an uniform dull yellowish tinge. The sides are only mo- 

 derately unequal ; the ventral edge is much arcuated, and 

 ascends the more on the shorter side. Both extremities are 

 rounded (they are not very unequal in breadth) ; that of 

 the longer side is a little more symmetrically and taperingly 

 so. There appears to be more curvature of the hinder 

 dorsal edge (and consequently less angularity at the upper 

 posterior corner) than exists in pusillum. The umbones 

 are tumid, obtusely rounded, and slightly prominent. 



There is an elongated and swollen variety, the form of 

 which is triangular or ovate-trigonal, with the ventral mar- 

 gin extremely obtuse. It is of a blackish hue, more or less 

 stained with ochre-colour. 



Mr. Jenyns, whose authority we have chiefly followed in 

 our account of the Plsidia, and to whose courtesy, in the 

 communication of specimens and information we are highly 

 indebted, first published this species as a British animal. 

 We confess that we have experienced much difficulty in 

 determining the limits of this and pusilla, although the 

 small ventricose examples of the former, in which the mar- 

 ginal zone is well developed, and the umbones are promi- 

 nent, may be easily separated from larger-sized compressed 

 typical forms of the latter, where the colouring is uniform, 

 and the umbones not at all projecting. 



Ordinary specimens do not generally exceed the ninth 

 of an inch in length, but some arc occasionally obtained 



VOL. II. R 



