166 MYTILIDiE. 



basal area. This portion, which, as well as the remainder 

 of the surface, is covered with a glossy, olivaceous, ash- 

 coloured epidermis, exhibiting a more yellow complexion 

 upon the umbones, and in the immature specimens, is desti- 

 tute of markings, and so abruptly compressed that its 

 upper edge is distinctly angulated. Along that line the 

 valves, which may be termed cuneiform in convexity, are 

 extremely ventricose, but rapidly diminish above it. 

 Generally speaking, the basal portion is flat or concave, 

 and its outline is usually incurved, particularly near the 

 beaks, which are extremely acute, bent down a little, 

 almost touching, and usually decorticated. The texture is 

 opaque and rather strong ; the byssal aperture is of an 

 elongated or lanceolate form, and the sinus for it, is usually 

 more profound in the margin of one valve than in the 

 other. The general shape is that of the common mussel^ 

 and liable to the same modification in its proportions : we 

 have not, as yet, however, seen any individuals, which re- 

 semble the broad variety of that shell, and the basal part 

 of the hinder extremity seems always the most projecting, 

 since typically the upper or posterior edge sweeps to the 

 base in one arcuated and much declining curve. The 

 cardinal edge is short, occupying only one-fifth of the out- 

 line. The interior, which is of a milk or porcelain white, 

 and not distinctly iridescent, often displays the external 

 markings : the lamina, which connects the two sides of the 

 shell at the beaks is rather deep set, smooth on the sur- 

 face, and rather strong : there seems an obscure apical 

 elevation of the margin in the right valve, and a corre- 

 sponding sinus in the left. 



Individuals whose length is an inch and three-quarters, 

 and whose breadth is one inch, may be regarded as remark- 

 ably fine. The epidermis is often much wrinkled behind ; 



