34 MOLLUSCA FROM THE CRAG. 
identical, as suggested by Philippi, ‘En Moll. Sic.,’ vol. i, p. 84, notwithstanding a 
material change in its habits appears to have taken place in the modern shell, where, 
after it has attained a certain age or magnitude, it attaches itself to some stone or 
rock, by the entire surface of the right valve adhering by the scabrous or imbricated 
portions of the rays, in consequence of which it is often distorted, moulding itself to the 
inequalities of the body to which it is fixed, producing great variation in form, from 
which circumstance the recent shell has been separated into two or more species. 
This does not appear to have been the habit of the Crag shell, as amongst the ~ 
numerous specimens I have seen, there is no indication of its having been attached by 
the exterior surface, but may have been a fixed species by means of a byssus, as in 
all the right valves a large opening exists beneath the anterior auricle, so far 
resembling the habits of the recent species in being fixed though by a different process.* 
Some of my specimens have attained a diameter of rather more than 2} inches 
from the umbo to the ventral margin, but, unlike the full-grown recent shell, 
have retained their regularity of form throughout their whole existence. There is 
often a slight obliquity in the shell, produced probably from its attached habits, the 
large byssus causing the auricle on that side to be more elevated than on the 
posterior, which is not only much smaller, but more depressed. The valves are nearly 
equal in convexity, though the right one is alittle the flatter of the two, and in general 
the rays are arranged in pairs, unequal in size, the larger one being scabrous, sometimes 
less regular with three of different sizes, and all imbricated, the exterior is, however, 
very deceptive in this character, as in one variety which from the entire absence of 
these imbrications as well as from a greater regularity in the rays, it was assumed to 
be a distinct species, and passed in my catalogue under the name of s¢reaturus, which 
there is reason now to believe is not entitled to that distinction. At the anterior 
opening beneath the auricles, are five or six elevated ridges for the purpose, probably, 
of keeping the byssus more spread, or in its place, and the shell appears to have 
had a somewhat large cartilaginous area, as well as a broad surface for the attach- 
ment of the ligament, the valves opening about five and twenty degrees. In the 
young as well as in the adult shell, there is a great equality in the size of the 
auricles. 
In this, as in many species of this genus, the exterior is ornamented with fine 
divaricating strize, crossing the lines of growth at nearly right angles, visible also in 
the recent shell, and between the rays the surface is often subcancellated by the 
reflexed or thickened margin of the shell at the varied periods of imcrease. 
The recent shell is stated, by the authors of British Mollusca, to range from near 
low water mark to 90 fathoms: most plentiful between 15 and 28. Its lateral range 
extends from the Mediterranean to the Norwegian shores. 
* Although never being itself fixed by the shell, its roughened exterior is well adapted for parasitical 
animals and we, consequently, find Oysters or Barnacles adhering to the valves. 
‘ 
