POWERS OF BURROWING. 845 



believe that in these cases, the valves had projected freely 

 beyond the cavity in which their peel ancles were lodged. 

 I may here also mention that in Mr. Cuming's specimen, 

 above alluded to, the basal cups of five specimens touched 

 and adhered to each other ; I was not able to make out 

 whether there had originally existed separate burrows, as 

 I think is most probable, and that the walls had been 

 wholly worn away, or whether the five specimens had fixed 

 themselves on one side of a large pre-existing, common 

 cavity. Young specimens seem to burrow to the full 

 depth, before nearly acquiring the diameter which they 

 ultimately attain. I measured one burrow, 1*2 of an 

 inch in depth, which, at its mouth or widest part, was 

 only *17 in diameter. 



The several species occur imbedded in soft calcareous 

 rocks, in massive corals, and in the shells of mollusca and 

 of cirripedes. It has been doubted by several naturalists, 

 whether the basal calcareous cup at all belongs to the 

 Lithotrya, but after the foregoing microscopical observa- 

 tions on its structure, it is useless to discuss this point. 

 So again it has been doubted whether the cavity is formed 

 by the cirripecle itself; but there is so obvious a relation 

 between the diameters of specimens of various sizes, and 

 the holes occupied by them, that I can entertain no doubt 

 on this head. The holes, moreover, are not quite cylin- 

 drical, but broadly oval, like the section of the animal. 

 The simple fact, that in this genus alone each fresh shelly 

 layer round the bases of the valves, and therefore at the 

 widest part of the capitulum, are sharply toothed ; and 

 secondly, that in this genus alone a succession of sharply 

 serrated scales, on the upper and widest part of the pe- 

 duncle, are periodically formed at each exuviation ; and 

 that consequently the teeth on the valves and scales are 

 sharp, and fit for wearing soft stone, at that very period 

 when the animal has to increase in size, would alone 

 render the view probable that the Lithotrya makes or at 

 least enlarges the cavities in which it is imbedded. 



Although it may be admitted that Lithotrya has the 



