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



abode in the old burrows of Pholas and Saxicava ; and it is pro- 

 bably owing to this habit that powers have been attributed to it 

 which it does not possess. From a similar habit, Kellia suborhi- 

 cularis has also been stated to excavate ; and it is not unlikely 

 that several other reputed borers have no better title to be so 

 considered. 



It may still be asked ; — if the armature be of this formidable 

 nature, how is it that Saxicava is entirely confined to calcareous 

 substances ? Why should it not likewise burrow in softer ma- 

 terials, such as wood and shale ? This may be answered by 

 another question — why do Teredo and Pholas striata always bore 

 in wood ? And why is not Saxicava itself found in shells of 

 other mollusks, as is frequently the case with Lithodomus ? for 

 certainly an acid solvent could dissolve the calcareous covering of 

 these animals as well as hard limestone. 



Some impulsive instinct is most probably the guidance in 

 these matters, leading each species to that substance best suited, 

 in some way or other, to the ceconomy of its life. This selection, 

 without an apparent cause, is observed everywhere in the wide 

 field of nature : we see it in the nests of birds, which in closely 

 allied species are frequently built of different materials ; and we 

 see it in a striking manner in the habits of the burrowing bees. 

 The carpenter-bees (Xylocopce) are well known to excavate in 

 wood ; there is a species, however, of an allied genus, the Antho- 

 phora retusa, which ^^ makes its nest not only in hard dry banks 

 but also in the crevices of walls, burrowing through the mortar, 

 and causing much damage by loosening the bricks." It cannot 

 be from want of power that this species does not penetrate 

 wood. 



In Saxicava there is also a mechanical cause which may have 

 something to do with the matter. It has been already stated 

 that the rubbing instrument is held by the attachment of the 

 byssus in contact with the substance to be excavated ; and as 

 the byssus is small, it is ill calculated to maintain its hold of soft 

 friable rocks, such as shale, which, on the coast of Northumber- 

 land, is frequently exceedingly brittle ; so much so that the Alga 

 seldom grow on it, and the Patellce rarely trust themselves to its 

 treacherous surface. Clavagella, however, appears to burrow in 

 soft substances as well as in hard ones : this is easily accounted 

 for by the fact, that the attachment of one of the valves to the 

 side of the burrow renders the support of a byssus unnecessary ; 

 and having an extensive fulcrum, this species can therefore exca- 

 vate in soft substances with as much facility as Pholas. 



There is another phsenomenon in the history of the mollusks 

 which appears intimately connected with the subject just dis- 



