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 a little 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 striatums, 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 inequality in the size of the 

 auricles. 



In this, as in many species of this genus, the exterior is ornamented with fine 

 divaricating striae, 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 

 refiexed or thickened margin of the shell at the varied periods of increase. 



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. 



