MODIOLA. 187 



diating lines ; epidermis thick and leathery, folding over the margin, 

 of a chestnut or pitchy-hrown color, smooth, glossy, and with radiat- 

 mg wrinkles before the ridge, where it is generally darker colored, 

 while the ridge is lighter colored. The groove for the ligament is 

 long and deep, resting upon a prominent rib. Interior pearly, of a 

 somewhat livid color ; muscular impressions large and deep ; bys- 

 sus colored like the epidermis. Length, four and one half inches ; 

 height, two and one quarter inches ; breadth, two inches. 



Inhabits deep water, and is thrown up on every shore exposed to 

 the open sea. It probably dwells upon a rocky or pebbly Ijottom, as 

 its byssus would find no attachment in mere sandy or muddy regions. 



This shell is well known on account of its size and universal dis- 

 tribution along our coast. It often attains to a great size, and is 

 not unfrequently seen six inches in length. Specimens are rarely 

 found which are not in some way distorted. Hence the shells 

 assume a great variety of form. The distortion seems to depend 

 upon two causes : first, the body to which they are attached by the 

 byssus, which may modify the form of the basal margin, rendering 

 it more or less arched ; and second, the frequent injuries sustained 

 by being dashed about by storms among the rocks which they in- 

 habit. They are the more liable to accidents of this kind, in con- 

 sequence of their affording attachment to the Laminaria, and other 

 large sea-weeds, which, being acted upon by the violence of the 

 waves, tear the shells from their resting-places, and they are thus 

 dragged great distances. In fact, it is not usual to find a specimen 

 on the beach without some parasite attached. A common deform- 

 ity occurs at the posterior or broader end, by what would seem to 

 be an arrest of development. The growth does not go on in this 

 direction, the successive layers extending very little beyond each 

 other, and thus we have a broad, blunt termination. 



The Mytilus umbilicatiis of Pennant can be nothing more than a 

 distorted variety from some injury to the lower margin, nearly under 

 the beaks, or from adhesion to some small convex body, causing 

 great contraction at this part, as may be frequently noticed. 



In young and entire shells the hinge-margin rises in a straight 

 line to a considerable height, and then slopes downwards, suddenly 

 forming a conspicuous angle ; but in older shells the angle disap- 

 pears in a great measure, and the whole superior outline is regularly 

 curved. When young, the epidermis seems to be prolonged at the 

 lines of growth into fringe-like slireds. Specimens thus clothed 

 are generally allowed to be the Mytilus barbatus of Pennant. M, 



