ARCA. 239 



As Dr. Phillppi correctly observes, this is a peculiarly 

 variable shell, the chief difference of contour being effected 

 by the convexity or straightness of the hinder margin. In- 

 dividuals of the former shape (var. Quoyi), are generally 

 more compressed when young, are more or less distinctly 

 biangulated posteriorly, and have their radiating strise 

 finer and more closely disposed ; examples of the latter 

 form (var. Gaimardi) are usually more abbreviated in 

 shape, and have their beaks more approximate, their epi- 

 dermis more copious and of a softer texture. The valves 

 which are opaque, strong, equal, and subrhomboidal in 

 contour, are ordinarily ventricose in the adult, with their 

 convexity tolerably evenly diffused, and become more or less 

 flattened, but not concave, beyond the obtuse umbonal 

 ridge. The entire surface, which beneath the pale or yel- 

 lowish brown and somewhat squamose epidermis, is of a 

 dull squalid white or light buff" colour (dead valves are 

 snow-white) is closely radiated with most numerous raised 

 subgranulated strise, which are slightly stronger at both 

 extremities, and decidedly more distant behind. Their in- 

 terstices are either narrower or scarcely broader than these 

 costellar strise, and are devoid of any radiating transverse 

 striulse, but occasionally reticulated from the lateral pro- 

 jection of the granules. The anterior side, although de- 

 cidedly the longer, is not very greatly so, its extremity is 

 rounded below, and a little angular above ; the hinder 

 termination is more or less biangulated. The dorsal and 

 ventral edges are nearly parallel ; the latter rises rather 

 the more in front of the shell. The cardinal area is much 

 sunken, not variegated, rather narrow, and of no great ex- 

 tent ; the ligament itself is oblong-lanceolate, dusky, and 

 marked throughout with most delicate striulse, which run 

 parallel to each other between the scarcely inclined and not 



