196 MYTILID^. 



illustrious Swede, under the name discors. His descrip- 

 tion was very accurate, and more extended than was usual 

 in the " Systema" ; the somewhat puzzling assertion, that 

 the strife are almost transverse, arose from his regarding 

 the attenuated end, and the wide posterior termination of 

 the mussels as top and bottom. 



The valves which are subovate, manifestly dilating pos- 

 teriorly, instead of being swollen as in marmorata, are but 

 moderately convex, and are even shallow at the extre- 

 mities. Considering their thinness and fragility, they are 

 rather opaque, and are covered with a closely adherent epi- 

 dermis of various shades of green and olive, more usually 

 in northern examples of an olivaceous brown, generally 

 with a greenish cast towards the edge, and in southern 

 specimens of a somewhat tawny olivaceous green. The 

 shape of the former of these two varieties, we may remark, 

 is more abbreviated, and the dorsal line is rarely quite 

 equal, as in the latter and more produced form, to the 

 space behind it. Of the three subtriangular compartments 

 into which the surface is divided, the posterior is slightly 

 the more elevated, and is rather more than equal in extent 

 to the two remaining ones. Typically, it is crowded with 

 most numerous radiating rib-like strife, which are simple, 

 not crenated, closely set, but little raised, and, in the adult, 

 almost invariably grown obsolete, except near the ventral 

 margin. There are a few distinct radiating costellse upon 

 the anterior portion, which are rather broad, not much ele- 

 vated, and sufficiently apart : the central area is devoid of 

 any sculpture, and appears rather abruptly terminated 

 behind by the superior elevation of the posterior portion. 

 The general inclination of the ventral margin is not far 

 from straight, but its outline is rather flexuous in detail, 

 arching out more or less in the middle, and again, more or 



