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cimen in a manner different from that in which they are cemented in nature. 

 The collector, moreover, finds a piece of wood, or of hard clay, or of chalk, 

 or of coal-shale, or, even, of limestone or sandstone, riddled through and 

 through with Pholads, of all sizes, in the same relative position, and 

 wonders how any mollusk with so delicate and sharply sculptured a shell 

 could have bored its way into it. Here then is an instance in which a 

 knowledge of the general organization and habits of the animal, to say no- 

 thing of its anatomy, is more than commonly essential. In Pholas we 

 have a lamellibranchiate mollusk allied to the Mi/aria, but having its soft 

 parts and shell largely modified for the performance of the boring functions 

 assigned to it ; and so variable are these modifications, so variably does 

 nature shift to provide for this change of habit, that the characters of spe- 

 cies are almost as strongly marked, and as distinct from each other, as the 

 characters of ordinary genera. We shall presently see that the fifty known 

 species of Pholas may be distributed into at least eleven well-defined 

 sections. 



Seeing that the habit of Pholas is to bore into substances and live and 

 die in them, the conchologist should examine his specimen with this in 

 view. The anterior portion of the shell will be found to gape, in some 

 species abruptly and largely, for the passage of a foot well constructed 

 for the work it has to do. The foot is more than usually developed, and 

 even when seen in a retractile state through the pedal opening of the 

 mantle, may be observed to terminate in a kind of sucker. Though many 

 theories have been advanced to account for the mode in which the boring 

 operations are carried on, some persons alleging that the means are purely 

 chemical, others that they are purely mechanical, the process has never yet 

 been accurately determined. The balance of opinion is much in favour 

 of the mechanical theory, but it is more than probable that the process is 

 a combination of both. The cavities of the Pholas not unfrequently ex- 

 hibit marks of the attrition caused by the rotatory motion of the shell, a 

 motion to promote which the foot would seem to be specially constructed, 

 aided, it may be, in some cases by the presence in the skin of particles of 

 silica ; and the wall of the cavity is softened, doubtless, by a solvent or by 

 the action of currents of water, in such a manner that the shell may rotate 

 without damage to the prickly scales of its sculpture. But the foot has 

 another office to perform; it is reflected back over the hinge for the 

 secretion of callosities and plates. The natural position of Pkolas, like 

 Mya on the one hand and like Aspergillum on the other, is with the foot 

 downwards, and it consequently bores in that direction; the siphons, as 

 in those genera, are extended perpendicularly to some distance through 

 the lesser anterior gape of the shell, and they are also united in a simi- 

 lar, or nearly similar, epidermic sheath. The boring functions of the 

 Pholads are exercised chiefly in an early stage of growth ; generally, the 



VOL. II. Z 



