56 THE ZOOLOGIST. 
In regard to the borings of mollusks, many authors—such as 
Bonanni, Adanson, Born, J. HE. Gray, Fleming, Osler, Forbes 
and Hanley, Cailliaud and Robertson—have believed that such 
are made by rotations of the valves after the manner of augers. 
Little, however, is said about the smooth valves of such as 
Saxicava, Gastrochena, and Lithodomus in contrast with the 
rough valves of Pholas and T'eredo. 
This theory, however, is not supported by an examination of 
the perforations of the Alge, Sponges, Bryozoa, those of the 
Annelids, Gephyreans, Cirripedes, and Helix, nor by a compari- 
son of the shells and tunnels of the animals themselves. The 
texture of the valves of the date-shells, for instance, is so soft 
that they could not act materially on the hard stones into which 
they bore. Moreover, it has to be remembered that the surface 
of such shells is covered by the periostracum, which would mate- 
rially suffer before much effect could be produced on the rocks. 
Hancock, again, propounded the theory that the holes were 
made by silicious particles in the foot of the mollusk (which 
particles could not, however, be found by that most accurate and 
conscientious observer, the late Prof. Busk), while Bryson gave 
the chief action to grains of sand from the exterior. Fisher 
thought that they were effected by the foot in some way. None 
of these views would explain the perforations in limestone by 
the ‘‘ Hélices saxicaves’’ of Bouchard-Chantereaux.* Other 
authors asserted that the annelids bored by aid of their bristles, 
just as Darwin had predicated of the Gephyreans and their 
chitin. 
A more gentle method of tunnelling was that advocated by 
Garner, who held that the excavations were due to ciliary currents 
aided by rasping. The currents may assist, but seem to be in- 
sufficient to account for the borings in any group. 
The next theory is one that has been frequently applied to 
the action of marine animals on their surroundings, viz. that 
the perforations are due to a chemical solvent. Amongst others 
who have chosen this explanation may be mentioned Gray, 
Osler (for Saxicava), Drummond, Cailliaud, Mantell, Thorrent, 
** Ann. des Se. Nat. 4e sér. xvi. pp. 197-218, pl. 4. These figures exactly 
correspond with a fine mass of limestone perforated by Helia sent by Dr. 
Scharff from Ireland. 
