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 versd. 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 (J/. 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. 
