98 PHOLADIDvE. 



us not lose sight of the fact that the shells of many mol- 

 lusca which are not borers have also prickles or spines 

 like those which cover the shell of a Pholas. Anomia, 

 Pecten, Lima, Area, Cardium, Venus, and Psammobia 

 offer examples among the British shells of this sort of 

 ornamentation. Such asperities appear to result from 

 a superfluous secretion of shelly matter, which it is con- 

 venient to dispose of in this way ; they strengthen the 

 fabric of the shell, but are of no further service to its 

 constructor. To my desire of doing justice to the inves- 

 tigation of M. Cailliaud must be added an apology for 

 having, in the introduction to the first volume of this 

 work, misinterpreted his views as to the mode in which 

 the Pholades excavate rocks. The mistake was partici- 

 pated by Dr. Fischer, and arose from the following pas- 

 sage in M. Cailliaud's 'Memoire sur les Mollusques 

 perforants' (1856) "les siphons des pholades, coupes 

 en pieces et morceaux, attestent (dans les temps voulus) 

 la presence de cette liqueur acidulee. Elle est done 

 faible puisqu'elle ne parait sous aucun rapport nuire a 

 Porganisation de ces animaux, et cependant elle dissout 

 des test de coquilles tres-durs, les calcaires les plus com- 

 pacts." (p. 27<) I certainly understood by this, that 

 M. Cailliaud was of opinion that the Pholades employ 

 an acid or corrosive solvent in excavating calcareous 

 rocks; but he has since distinctly asserted that they 

 make use of their shells only. It had been generally 

 supposed that Pholas does not secrete an acid. M. 

 Thorrent, however, in the ' Journal de Conchyliologie ' 

 for 1850, proved that an acid exists in P. crispata; 

 and this important discovery has since been confirmed 

 by M. Cailliaud as to other species of Pholas. But how 

 do the fry perforate ? Are we to suppose, with M. Cail- 

 liaud, that the parent makes with its acidulated siphons 



