GENUS VERRUCA. 515 



excavation of the support does not depend on its hardness, 

 bat on its containing calcareous matter, liable to be acted 

 on by some solvent : but as this view, considering what we 

 know of Lithotrya and of the two other burrowing genera 

 of cirripedes to be hereafter described, appears improbable, 

 I will add a few additional observations. I most carefully 

 examined the shell and basal membrane of Verruca, and like- 

 wise the tissues left after the dissolution of the shell in acid, 

 and could detect no structure at all fitted for boring ; and 

 what appears more important, there was no apparent differ- 

 ence in the state of the specimens which had and had not 

 excavated a hollow ; and this, I think, would certainly have 

 been the case (as in Lithotrya) if the action had been 

 mechanical. It is not easy to ascertain, owing to the 

 small effect at any time produced, at how early an age 

 Verruca begins to act on its support ; but I found two sets 

 of specimens only ^th of an inch in basal diameter, which 

 had certainly commenced. The ribbed shell, (PL 21, fig. 6), 

 especially the middle rib, shows, in a somewhat exaggerated 

 degree, the typical form of the excavation ; it may be here 

 seen that the excavation is of the same depth for some 

 little distance from the circumference towards the centre, 

 but that in the middle it suddenly becomes deeper. I 

 have seen several specimens with a central hollow, with- 

 out any, or with scarcely any, marginal depression, and 

 likewise the reversed case. These several facts show that 

 the central excavation cannot be due to an equable action, 

 prolonged during the whole growth of the shell, having 

 thus affected the middle more than the circumferential 

 parts, for in this case the excavations would have sloped 

 into each other. In specimens which have not at all acted 

 on their support, the whole basal membrane is firmly attached, 

 as in all ordinary cirripedes, to the supporting surface ; but 

 in those which have acted, the middle portion of the basal 

 membrane is quite unattached, and the circumferential 

 portion is, I think, less firmly attached than is usual ; but 

 between these two portions, there is a circular zone strongly 

 cemented to the supporting surface, and which alone keeps 

 the shell in its place. Now, on the mechanical theory, to 

 account for the circumferential hollow, the basal edoes of 



