206 MYTILID^. 



and exhibits a difFercnt proportion in its component parts. 

 We must preface our description of this shell by observing, 

 that we have seen too few specimens of it to determine 

 whether the characters we have noted and delineated 

 should be regarded as essential, or as liable to alter- 

 ation. Of its distinctness as a species, we entertain 

 no doubt. The valves are small, oblong, or elongated ob- 

 long, and much narrowed in front ; they are moderately 

 thin, ventricose, and covered with a lustrous ej^idermis of a 

 clear semi-transparent bluish green, beneath which the shell 

 is variegated with zigzag markings of a reddish liver colour 

 on a paler ground ; this marbling is not partial or confined 

 to the umbones, although more profusely displayed there, 

 or from the greater tenuity of the epidermis at that part, 

 more distinctly apparent upon them. The external surface 

 is radiatingly divided into three areas, of which the hinder, 

 although occupying twice the actual superficies of the other 

 two united, does not extend at the ventral margin to quite 

 the middle of it, although advancing further than in the 

 two sj)ecies previously referred to. This area, which in our 

 most characteristic examples is abruptly elevated above the 

 general level, is closely radiated with, at least, twenty 

 sulci, whose terminal interstices in the larger individuals 

 are elevated into distinct costellse, with faint and close con- 

 centric strise in their intervening furrows. The few de- 

 pressed ribs which occupy the anterior area are broad and 

 rather far apart, their interstices about equalling them ; 

 the line of demarcation between them and the smooth and 

 larger central area is not defined. At the junction of the 

 front and middle areas, the ventral margin, whose general 

 outline is straightish, is a little retuse ; it swells out again a 

 little in the centre, becomes again retuse, and finally, by its 

 sudden projection along the base of the hinder area, gives 



