514 VliRRUCIDiE. 



thickness of hard shell ; but the thinnest film of any matter 

 on which acid does not act, as of grease in certain forms of 

 printing, will perfectly preserve the underlying substance, 

 and as I have ascertained by putting on a drop of acid, is 

 the case with this membrane. I have removed several 

 scores of shells of V. Stromia from the stems of Laminariae, 

 and when the latter were washed and slightly dried, gene- 

 rally not the least effect could be seen, except that the spots 

 where the shell had adhered were glossy from the still 

 adherent basal membrane : yet the stems of Laminariae are 

 far from hard. In some cases, however, the attachment 

 of the Verruca seemed to have produced a very slight 

 depression on the Laminaria, but this, I think, may be 

 safely attributed to the growth of the surrounding sur- 

 face ; for I have seen exactly the same effect produced by 

 the attachment of the discs of the antennae of a Lepas, 

 and these discs, with their long spines, could not possibly 

 produce any excavation ; nor is Lepas or its pupa in any 

 case a burrowing animal. Again, I have seen a few 

 specimens of Verruca attached to Gorgoniae, and they had 

 not acted in the least on the horny axis. I have examined 

 numerous specimens of V. Stromia attached to three pieces 

 of slate-rock, and to one piece of red sandstone, all from 

 different localities, and no effect whatever had been pro- 

 duced ; yet the slate-rock, especially in one instance, was 

 soft. Mr. Bate, to whom I am indebted for some of these 

 specimens, also informs me that he could discover no im- 

 pressions on the slate-rocks, whence specimens of the 

 Verruca had been removed. On the other hand, I have 

 had two specimens of limestone, with attached Verrucas, 

 one coarse and very impure, and the other hard and 

 marble-like ; and in both cases there was a distinct central 

 slight cavity, including loose gritty matter. The loose par- 

 ticles evidently resulted from the unequal action either of 

 a solvent or of some mechanical power on the rock, for it is 

 improbable in the highest degree that the shells should have 

 fixed themselves exactly over small collections of loose par- 

 ticles, even if such could possibly have remained on pro- 

 jecting surfaces of sea-washed rocks. 



The above facts seem to indicate pretty plainly that the 



