PIDUOCK. 101 



which was performed by gently turning the shell from right to 

 left, and back again. 



The Pholas burrows to a considerable depth, and if a piece of 

 the rock be detached and broken to pieces by the hammer, it 

 wiU be seen to be completely riddled with the perforations. 

 Chalk-rock is mostly the richest in specimens, but even the hard 

 limestone formations are penetrable by the fragile shell of the 

 Pholas. It has been well remarked, that the size of the Pholas 

 and the sharpness of its markings vary in inverse ratio to the 

 hardness of the rock in which it burrows. From the softest sea- 

 beds are taken the largest and most perfect shells, whUe those 

 specimens which are obtained from the hard limestone rocks, 

 are comparatively small, and the surfaces are rubbed nearly 

 smooth. The very worst examples, however, are those which 

 are found in gritty rocks, interspersed with pebbles. The shells 

 that have burrowed into such substances are dwarfed, abraded, 

 and often misshapen, and are valueless except to the physio- 

 logist. 



We naturally ask ourselves why the Pholas should bury itself 

 in the rocks instead of passing its life in the open sea, like 

 the generality of bivalves. The creature does not feed upon the 

 substances in which it forms its curious tunnel, and to all 

 appearance would obtain as much food without as within the 

 burrow. One obvious answer to this question is, that the 

 creature buries itself in the rock for the sake of safety, its shell 

 being, as has already been remarked, of a soft and fragile texture. 

 This opinion is further corroborated by the fact that one of the 

 British species, the Paper Pholas {Pholas jpajn/racea), has a pecu- 

 liarly thin and delicate shell, so as to earn for it the name by 

 which it is popularly known. Yet, although this may be a 

 reason, it is not the reason; for there are many well-known 

 shells which are far more fragUe than those of the Pholas, yet 

 which need no such protection, and instead of concealing them- 

 selves in any way, roam the ocean freely. 



In my own opinion, the burrowing instincts of this and many 

 other marine creatures of similar habits, are implanted in them 

 for other than mere individual purposes. Judging by the effects 

 which these animals must have produced upon the line of coast 

 throughout a succession of centuries, I cannot but think that 

 they are, at all events partially, intended as instruments which aid 



