124 INVERTEBRATA OF MASSACHUSETTS. 



the passage of the byssus. From the beaks a very convex, broad 

 ridge runs diagonally across the shell ; above this the shell is com- 

 pressed, and along its lower and anterior side is a broad depression 

 or constriction, terminating at the lower margin where the shell 

 gapes. Surface roughly marked by the lines of growth, and by a 

 few faint, radiating lines ; epidermis thick and leathery, folding 

 over the margin, of a chestnut or pitchy-brown color, smooth, 

 glossy, and with radiating 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 ; mus- 

 cular impressions large and deep ; byssus colored like the epider- 

 mis. Length 4| inches, height 2^ inches, breadth 2 inches. 



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

 to the open sea. It probably dwells upon a rocky or pebbly 

 bottom, 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 inhabit. They are the 

 more liable to accidents of this kind, in consequence of their affording 

 attachment to the Lamindria, 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 deformity occurs at the posterior or broader 

 end, by what would seem to be an arrest of developement. 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 ter- 

 mination. 



The M. umhilicdtus of Pennant can be nothing more than a dis- 

 torted 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. 



