THE BORING MOLLUSCA. 161 



umberland, where there is abundance of soft shale and 

 a great variety of rocks, it certainly never does so : but it 

 frequently takes up its abode in the old burrows of Pholas 

 and Saxicava ; and it is probably owing to this habit that 

 powers have been attributed to it which it does not possess. 

 From a similar habit, Kellia suborbicularis 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 for- 

 midable nature, how is it that Saxicava is entirely confined 

 to calcareous substances ? Why should it not likewise 

 burrow in softer materials, 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 Saxi- 

 cava 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 economy of its life. 

 This selection, without an apparent cause, is observed every- 

 where 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 

 (Xylocopae) are well known to excavate in wood. There is 

 a species, however, of an allied genus, the Anthophora 

 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 attach- 

 ment 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 Northumberland, is frequently exceedingly 

 brittle ; so much so that the Algae seldom grow on it, and 

 the Patella rarely trust themselves to its treacherous sur- 

 face. Clavagella, however, appears to burrow in soft sub- 

 stances 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 



M 



