162 GASTEROPOD BORERS. 



unnecessary ; and having an extensive fulcrum, this species 

 can therefore excavate in soft substances with as much 

 facility as Pholas." * 



There are no borers amongs the tunicated, brachiopod, or 

 cephalopod mollusca; but a few of the Gasteropods have 

 the same power, and it has been attributed to others, on 

 rather uncertain grounds. The Limpet (Patella vulgata) 

 seems to be in the habit of hollowing out a space in the 

 site on which it has settled, answering to the size and 

 shape of the rim of the shell ; and it does this probably 

 with the intention of obtaining a stronger seat or hold with 

 the expenditure of less muscular power, or rather, with the 

 relaxation of all muscular contraction, so that the shell may 

 be elevated sufficiently to admit the influx of water around 

 the branchiae without any danger of the animal being driven 

 from its settlement. The excavation varies in depth, not 

 according to the chemical composition, but according to the 

 softness of the site, from a line to half an inch ; but the 

 shell is never buried in it. Mr. J. E. Gray, who first called 

 particular attention to this peculiarity in the limpet's habits, 

 explained the operation as the result of a solvent fluid, 

 excreted from the sole of the foot, of which, however, no 

 evidence was produced. De Montfort came near the truth. 

 Some Patellae, he says, when in a state of repose have a 

 peristaltic motion in the foot, which slowly hollows out the 

 stone on which they rest by friction alone, and these species 

 are sedentary.f " By friction alone " the operation is done, 

 and the foot is armed with hard crystalline siliceous spicula, 

 similar in all respects to those of the perforating instrument 

 of the Pholas, and replaced by new ones as often as the 

 friction has rubbed off the asperities of the old. 



The questionable Gasteropod borers are our common snails. 

 I shall give you all the evidence I possess, and, curiously 

 enough, the question has become connected with some inter- 

 esting geological phenomena, which lie, however, beyond 

 our demesne. " On the east side of Whelpington," says 

 the Rev. Mr. Hodgson, in his " History of Northumberland," 

 " a stratum of limestone is here and there seen in grey pro- 

 jecting masses, the under-surface of which is bored upwards 

 into cylindrical holes, which are from a line to four inches 

 deep, and tenanted, especially in winter, by the banded and 

 yellow varieties of the Helix nemoralis. The Limax, while 

 it occupies these cavities during the summer, has its fleshy 



* Ann. and Mag. N. Hist. ser. 2, ii. 242244. 

 t Conch. Syst. ii. 68. 



