PIIOLAS. 97 



in the Memoirs of the French Academy for 1812. He 

 figures the Pholas candidus in its cavity, and attempts to 

 account for its presence there. He remarks that it is al- 

 ways found in cavities, either of soft stone or clay ; that 

 these are made by the efforts of the animal itself, and by 

 means of its foot, for when it was placed by him upon soft 

 clay, it buried itself in that substance by the action of its 

 foot. He argues that they bore only in soft clay, and that 

 their presence in stone (soft stone, which he terms "la 

 Banche,") is owing to the former having petrified around 

 the Pholades. He shews that the dimensions of the cavity 

 in which the full-grown Pholas is found are, as compared 

 with shell and aperture, such that the former must have 

 remained in it since it first perforated, and could not have 

 changed its habitation. He states that the young are al- 

 ways found in clay, and the old in stone, and concludes 

 that the stone is only clay petrified by means of a viscous 

 matter derived from sea-water. It need not be said now 

 that Reaumur's observations and conclusions were falla- 

 cious, but as a first step in the inquiry they had great 

 merit. 



Mr. John Edward Gray, in an interesting paper on the 

 habits of Mollusca, published by him in the " Philosophical 

 Transactions," for 1833, gives his opinion on this question. 

 He holds that Pholades^ Petricola, Venerupis, and Litho- 

 domus, bore into shells and calcareous rocks by dissolving 

 them. His reasons for holding this opinion are several ; — 

 1st. because the animals of most of them are furnished 

 with a large foot more or less expanded at the end ; 2nd. 

 because the holes fit the shell — in Petricola and GastrocJiana 

 — so as to prevent rotation, and the use of the asperi- 

 ties on its surface ; 3rd. because all borers are covered with 

 a periostracum, (thin in Teredo, Pholas-, and Lasea ; thick 



VOL. I. o 



