GENUS VERRUCA. 517 



it is strongly cemented? On the other hand, on the 

 theory of a solvent slowly poured out from the cement- 

 ducts, its separation from its support is simply explained. 

 It might be supposed that the calcareous matter, when 

 dissolved, would not be able to escape from the central 

 hollow, owing to the basal membrane being so firmly 

 cemented all round it ; but the attachment is by a 

 reticulated layer of cement ; and I infer that it must be 

 permeated by open passages, from the fact of the hollow 

 being often filled, in dried specimens, by a bubble of air, 

 instead of the basal membrane being pressed closely down 

 into the hollow, as would have been the case had the hollow 

 been hermetically sealed up. I have seen a few instances in 

 which the bottom of the central hollow was occupied, (as 

 was remarked to me by Mr. Hancock), by a little chalky and 

 gritty matter ; and in the case of one of the specimens of 

 calcareous rock, before alluded to, by coarse grains and 

 oxide of iron ; this seems quite compatible with a solvent 

 acting more readily on certain parts of the rock or shell 

 than on other and less soluble parts or particles. 



The greatest depth of the central hollow, in any specimen 

 seen by me, even measuring from the top of a rib in the 

 case of a ribbed shell, to the deepest point, was only 5 ' 5 th of 

 an inch; but considering how much depressed the shell of 

 Verruca is, I have no doubt that this small gain of space is 

 of service to the animal : we must suppose the loose middle 

 portion of the basal membrane is stretched slightly, or splits 

 and is repaired, so as to fit the hollow. With respect to 

 the even much slighter circumferential excavation, it barely 

 equals in depth the thickness of the extreme edges of the 

 walls ; it must, I presume, give strength to the shell when 

 laterally pushed ; but it certainly appeared to me that the 

 individuals which had excavated a depression for themselves, 

 could be prised vertically up much more easily than those 

 which had not acted on their support. Finally, we must 

 suppose that the hypothetical solvent is poured out of the 

 cement-ducts at the extreme circumference of the basal 

 membrane, which is almost loose and destitute of cement, 

 so as to slightly corrode outwards and downwards the cal- 

 careous support ; the action here then stops, and this rim 



