234 Mr. A, Hancock on the Boring of the Mollusca into Rocks, 



Some of the Gasteropoda, — Patella, Hipjionyx and others, are 

 well known to make holes in rocks and other hard substances. 

 On the coast of Northumberland Patella vulgata occasionally 

 sinks half an inch into the softer kinds of stone ; and Mr. Garner 

 asserts that it often forms holes an inch deep. Mr. John Edward 

 Gray, in his valuable paper on the (Economy of the Molluscous 

 Animals, published in the ' Philosophical Transactions ' for 1833, 

 expresses an opinion that these holes are produced by the dis- 

 solving powers of the animal, not having observed Patella or the 

 boring Acephala to penetrate any but calcareous substances, 

 and those cemented by lime, wood and clay excepted. At Cul- 

 lercoats, however, where there are rocks of various kinds, Patella 

 does not confine its operations to such as contain lime ; it sinks 

 equally deep into shale and shaly sandstone, and even slight im- 

 pressions are occasionally made into the less compact siliceous 

 sandstones. The idea of a solvent would, therefore, appear un- 

 tenable. Patella certainly does not bore into the compact sili- 

 ceous freestones ; even the very hard limestones are never deeply 

 penetrated by it : into the softer rocks above-named it sinks 

 deeper, and still deeper in proportion to their softness. 



Mr. Garner, in following up his views, maintains the opinion 

 that it is by the action of the ciliated branchiae that these exca- 

 vations are made. But on examining them we find that there 

 is generally an elevated ridge corresponding to the space between 

 the mantle and foot ; in which space the gills are placed, and 

 where, of course, the currents are strongest. Were these exca- 

 vations effected by the branchial currents, there ought, certainly, 

 to be a depression in the place of this ridge. 



The holes made by Patella vary considerably in different ma- 

 terials. In hard stone not more than a mere circle, correspond- 

 ing to the edge of the mantle, is produced : in soft limestone the 

 mantle sinks to some depth, leaving the portion on which the 

 foot rests projecting up in the centre, sometimes more than one- 

 eighth of an inch. In the sound shales the bores are frequently 

 almost flat ; but in those that are friable, the foot, by its strong 

 grasping powers, tears up the stone, and thereby makes a deep- 

 ish cavity in the centre of the burrow : the surface of this cavity 

 has a rough and broken appearance, and is surrounded by a 

 smooth elevated rim sunk a little beneath the general surface of 

 the stone : this rim is produced by the mantle*. 



* The form of the shell of Patella is modified in consequence of this di- 

 versity of the burrows. When they are flat, the shell attains its normal 

 growth ; when the centre is elevated and the margin corresponding to the 

 mantle depressed, the shell becomes very much raised : but it is flattened in 

 a most extraordinary manner when the foot sinks deep into the centre of 

 the excavation. 



