184 MYTILIDJE. 



again swells out a little, and then diminishes again in front. 

 The chief compression is, of course, at the juncture of the 

 hinder dorsal edge with the posterior margin, which latter 

 being always more or less produced, the dorsal wing, which 

 is more or less subangular (the angle being occasionally 

 also rounded off), consequently lies just midway between the 

 umbones and the rather broad and well rounded termina- 

 tion of the hinder side. The cartilage margin, which is not 

 bordered by any dull strip of epidermis, as in Metcalfei, is 

 nearly rectilinear, but with a tendency to convexity, and 

 rises with an ascent sufficient to prevent the upper and 

 lower edges being at all parallel ; the upper posterior out- 

 line varies from somewhat arcuated to rectilinear, and is 

 never incurved or decidedly refuse. Owing to the all but 

 terminal position of the beaks, the anterior side is almost 

 rudimentary ; it is very narrow, but not attenuated below, 

 as in certain Modiolte, by any peculiarly oblique rise of the 

 ventral margin, but is rounded and about equally project- 

 ing above and below : the front dorsal edge is not on the 

 same plane with the hinder one, but decidedly below its 

 level. The filaments of the epidermidal beard, which is 

 chiefly apparent upon the younger shells, are very slender, 

 greatly produced, and not serrated on either edge. The 

 interior of the shell is whitish, but often slightly tinged with 

 lilac, a deep stain of which is generally visible externally 

 on removing the cuticle ; the hinge-margin does not exhibit 

 any teeth or crenulation, but is thickened at the beaks. 



The largest individuals usually come from the North, and 

 average about five inches in length, and rather more than 

 two in breadth : a specimen, measuring seven inches in 

 length, and three inches and a quarter in breadth, is re- 

 corded by Captain Brown to have been caught by a fisher- 

 man's line, near the Bell Rock, on the coast of Forfarshire. 



