300 ANATOMY OF THE LAMELLIBRANCHIATA. 



rocks are bored when soft ; but this certainly, in many cases, 

 cannot be the case. l They who believe the phenomenon to 

 be accomplished by the action of the valves, differ as to the 

 way in which they suppose them to act ; some thinking it is 

 by a filing, others by a rotatory motion. All animals which 

 have the power of exciting currents in the water, appear to 

 be able to excavate the most solid materials when they are 

 exposed to the action of such currents ; thus other animals, 

 besides the Lamellibranchiata, have such a power. The 

 Patella, for instance, when sticking to a rock of soft texture, 

 forms a hole or pit, sometimes an inch in depth ; and this it 

 appears to do by the action of the streams of water, brought 

 in by the circle of branchial processes, situated around the 

 foot, a cast as it were of which may be seen on the floor of 

 the cavity. This hole cannot be made by the shell, as it fits 

 exactly in it, and is of a figure such as to allow of no rotation. 

 The Hipponyx, 7 - another similar gasteropode, forms cavities 

 in the Patella and other shells to which it adheres. In the 

 piercing bivalves we always find the apices of the valves in a 

 particular direction, being constantly superior when the bore 

 is inclined ; and it is certain there can be no rotation, as the 

 cavity, in many instances, does not admit of it. The crypts 

 of the Saxicava, for instance, are not circular ; hence M. de 

 Bellevue and Osier in this instance, suppose them to be formed 

 by the action of the phosphoric acid secreted by the animal, 

 and they suppose that genus to inhabit only rocks composed 

 of carbonate of lime. But it may be asked, in opposition, 

 how the valves of the animal themselves are preserved un- 

 touched ? Neither is it true, as the author of this has con- 

 vinced himself, that the Saxicava is found only in chalky and 

 limestone rocks. The valves are often rounded at their ex- 

 tremity, or so thin and fragile as to be ill adapted for mecha- 

 nical action. When, as in the Pholades, the anterior extre- 

 mities appear more suited for such an effect, they,- on inspec- 

 tion, commonly present no appearance of having been worn 

 by such an usage ; on the contrary, their processes seem quite 

 perfect, whilst the sides of the valves, from the sliding of the 

 animal in its cell, are often nearly worn through. It appears 

 then that the mechanical apparatus of the different boring 

 animals of this class is insufficient to account for their power 

 of excavation ; and we must attribute it principally to the ac- 



1 The history of the temple of Serapis at Pozzuoli has been brought for- 

 ward on this point, by many authors. There can be no doubt that it has 

 been inundated by the sea, at which time its pillars were bored as they are 

 now seen. See Lyell, Geology ; and Stark, Brewster's Edin. Journ. vol. v 

 2 Such specimens may be seen in the British Museum. 



