THE SNAIL BORERS. 163 



longitudinal disk protruded out of the shell, and coiled 

 nearly into a circle on the surface of the stone, the summit 

 of its shell hanging downwards ; and in this position it pro- 

 bably elaborates its den in the same manner that some of the 

 Pholades work their way into clay and wood, or, by a slow 

 but constant process, sink and enlarge their cells in the 

 hardest stones." * In the subsequent page, the Reverend 

 annalist attributes the same property to Helix (Limnaeus) 

 putris, and makes it the agent of holes in the bed of the 

 little river Wansbeck. The proof, you will observe, here 

 led for the Helices being the operators is very inconclusive ; 

 and I shall leave the following, made by a more scientific 

 observer, to your own judgment: " During the meeting of 

 the Geological Society of France, at Boulogne, in September, 

 1839, Dr. Buckland's attention was called by Mr. Greenough 

 to a congeries of peculiar hollows on the under-surface of a 

 ledge of carboniferous limestone rocks. They resembled, 

 at first sight, the excavations made by Pholades, but as he 

 found in them a large number of the shells of Helix aspersa, 

 he inferred that the cavities had been formed by snails, and 

 that probably many generations had contributed to produce 

 them. 



" A few years since (viz., previous to 1841), the Rev. N. 

 Stapleton informed the author that he had discovered at 

 Tenby, in the carboniferous limestone on which the ruins of 

 the castle stand, perforations of Pholades thirty or forty feet 

 above high-water level ; but having recently examined the 

 spot, Dr. Buckland ascertained that these excavations were 

 the work of the same species of Helix as that which had 

 formed the cavities in the limestone near Boulogne, and he 

 found within them specimens of the dead shells as well as of 

 the living. The mode of operation by which the excavations 

 were made, he conceives, is the same as that by which the 

 Common Limpet (Patella vulgata) corrodes a socket in cal- 

 careous rocks, and he is of opinion that the corrosion is due 

 to the action of some acid secreted from the body of the 

 limpet or helix." A little afterwards the celebrated pro- 

 fessor observes, that the snails " could find shelter only on 

 the margin and lower surface of the projecting rock, and the 

 irregular form of the confluent cavities correspond with that 

 of the clusters of snails in their ordinary habitat and hyber- 

 nation ; and if to those reasons be added, the fact of finding 

 both living and dead shells in the excavations, the evidence, 

 the author conceives, is decisive as to the agency of snails in 



* Hist. North, part ii. vol. i. 193. 



M 2 



