54 MOLLUSCA FROM THE CRAG. 



form of this species, the other varieties have all been obtained from the older 

 formation. 



The ligament of this is placed within the margin of the shell, although slightly 

 visible externally, when the valves are closed, it extends the entire length of the dorsal 

 edge, and a considerable scope is given to the dilatation of the valves ; four small 

 teeth are placed on the anterior side of the umbo interlocking each other ; these teeth 

 are always visible, though somewhat variable in their character, the anterior one being 

 sometimes the larger, and vice versa. In the increase of the valves a slight curvature 

 is given to the umbo by the retrocession or retreating of the ligament : while fresh 

 layers are deposited on the anterior margin, three small ridges are left upon the 

 exterior, indicating the form and position of these teeth, which are produced internally 

 by the indentures of the exterior. A small but deeply-seated muscle mark is visible on 

 the anterior side, or immediately beneath the umbo, and a large subcircular one a little 

 within the posterior part of the ventral margin. Mr. Alder says, " no species under- 

 goes a greater degree of variation from locality than the common Mussel. For its 

 full development, a mixture of fresh with salt water appears to be necessary, it is 

 therefore met with in the greatest perfection at the mouths of rivers. In such 

 localities the typical form of the species is to be found, and when left undisturbed 

 usually forms large beds. On the more rocky and exposed parts of the coast it 

 assumes a stunted appearance, running into the varieties of form mentioned above, 

 always small on the exposed surface of rocks, but attaining a larger size in hollows 

 and crevices." 



In the Estuary Deposit of the Eocene Period, at Colwell Bay, as well as upon the 

 opposite side of the Solent at Hordwell, is found a species of Mussel (M. affinis, ' Min. 

 Conch.,' T. 532, fig. 1), which very much resembles the var. pellucidus of this species, 

 but it is decidedly more carinated, and wants the denticles so conspicuous in the 

 common edible Mussel, near the umbo ; and notwithstanding the extraordinary range 

 in variation assumed by this species, there is, I think, no doubt of the two shells being 

 specifically distinct ; a specimen from Bridlington, with this name, was obligingly sent 

 to me for description by Mr. Bean, but there is every reason to believe it is only a 

 variable form of our common Protean shell. 



It is, in general, of littoral habits, being often found in the living state where left 

 dry by the retiring tide, and as such, indicative of shallow water, although it is 

 occasionally met with at considerable depth. Its geographical range is very great, 

 being undoubtedly an inhabitant of the Mediterranean, as well as of the coast of the 

 United States of America, and in both of which extremes of longitude it appears to 

 be subject to the same variable character. This is a long known species, descriptions 

 or figures of the recent shell having been given by almost every author, ancient or 

 modern, who has ventured to describe a shell ; and in order to show its range in 

 variation, a list of names is introduced, under which it has been described, pre- 

 suming all to belong to one and the same species, a single example of each name 

 being considered sufficient for the living shell. 



