GENUS VERRUCA. 513 



have come to the conclusion that the excavation must be due 

 to a solvent, probably poured out from the cement-ducts, 

 which debouch on the under side of the basal membrane. 



In the first place, an epidermis, as just stated, perfectly 

 preserves the shells of the various species of mollusca and 

 certain cirripedes, to which I have seen Verruca attached : 

 this is well shown by comparing the effect produced on the 

 same shell in parts covered by the epidermis and in parts 

 whence it has been abraded; or where the shell of the 

 Verruca had fixed itself, whilst very young, within a crack 

 in the epidermis, and had subsequently, by its growth, 

 turned up the edges, and had then acted on the underlying 

 shell ; whereas the specimens attached to the sound epi- 

 dermis had not produced the smallest effect. Again, I 

 have seen an epidermis-covered mussel-shell encrusted by a 

 hard nullipora, on which V. Icevigata was attached ; and 

 here the calcareous nullipora, under the middle of the 

 basal membrane, was entirely corroded away, whilst the 

 underlying epidermis and the shell beneath it, were not in 

 the least affected. The protection afforded by the epider- 

 mis is still more strikingly shown by contrasting shells 

 with very sharp prominent ridges, when thus invested and 

 when naked, to which Verrucse have been attached : I have 

 given a figure (PL 21, fig. 6) of a piece of an invested 

 Venus, from the surface of which a V. Bpengleri had been 

 just removed ; on the other hand, I have seen a Peruvian 

 Discina in which even sharper ridges, covered by epi- 

 dermis, were left absolutely untouched, although pro- 

 jecting deeply into the shell of an attached V. l&vic/ata. 

 I have seen several specimens of this latter Verruca (which 

 has the power of corroding naked shell as deeply as its 

 congeners), attached to the membrane-covered variety of 

 Balanus Icevis, the shell of which was thus perfectly pre- 

 served : now this membrane is little more than the ^gtk of 

 an inch in thickness ; it is not hard, and so brittle that it 

 generally separates with the Verruca, leaving the under- 

 lying shell of the B. Icevis with its lines of growth glossy 

 and perfect : it appears to me impossible that a membrane 

 so thin and brittle could resist an action, if mechanical, 

 which has worn away from twenty to forty times as great a 



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