172 MYTILIDJE. 



with the dorsal angulation much more marked, and the 

 hinder termination more expanded than in the ordinary 

 cramped variety. In the latter, the cardinal edge is short, 

 and the posterior produced comparatively straight, and 

 running parallel to the ventral ; in the former the hinder out- 

 line arches out more boldly both above and below, and the 

 greater declination of the posterior edge renders the pro- 

 jection of the dorsal angle more apparent. This angle, 

 which is either subcentral or lies rather before the middle 

 of the shell, is greatly modified in the extent of its obtuse- 

 ness by the greater or lesser curvature of the upper or pos- 

 terior margin, and the degree of its declination. If the 

 angle formed by the junction then of the upper and cardinal 

 edges be a very obtuse one, the former will run nearly pa- 

 rallel to the basal, and instead of uniting with it in a single 

 sweep, generally forms a second very obscure rounded-off 

 corner, as it merges into the posterior extremity, which is 

 usually rather broadly rounded, and more projecting below 

 than above. The basal or front ventral margin, is by no 

 means constant in its outline ; occasionally as in the variety 

 incurvatus, it is profoundly concave, in others it is almost 

 straight ; in some it is slightly gibbous behind, in others it 

 swells out more towards the beaks. 



There are two principal variations of colour. That which 

 is the most common is where the shell, whose texture is 

 strong and nearly opaque, has the exterior, beneath the 

 olivaceous or yellowish-brown epidermis, of an uniform 

 blackish-blue, (the colouring matter in its earlier coats is 

 almost always arranged in darker rays upon a paler ground, 

 and in darker and lighter zones, as may be observed where 

 the surface is abraded); the basal portion, however, is often 

 of a squalid white ; the interior is of a more or less dull 

 white, margined with blue. The other, which is a far 



