THE SAXICAVJE THEIR HABITS. 155 



dual to take a short turn back in form of a siphon, rather 

 than work any distance across the grain. 



But the Lithophaga and Lithodomi (Fig. 26 a) have no 

 organs for boring similar to those in the Pholas, and yet it 

 were reasonable to suppose for them a structure stronger 

 and more fully developed for the purpose, did they really 

 operate mechanically, seeing that the substances they dig 

 into are harder than those selected by the Pholas or Teredo. 

 This anatomical argument might be deemed sufficient of 

 itself to prove that the Lithophaga must work by the agency 

 of other means. Moreover, the texture of the shell is so 

 soft, that it could make no impression upon the stone 

 without being itself acted on ; and the effect of this would 

 be permanent, because superficial injuries of the shell are 

 never repaired. But nothing of this kind is met with. Mr. 

 Osier has even found a Saxicava 

 rugosa (Fig. 28, a), the species on Fig- 28. 



which his observations were made 

 fixed between two others, which 

 was so compressed that it was 

 quite flat, and little more than a 

 third of its proper thickness ; yet 

 neither of the three showed the 

 slightest mark of friction, and 

 the cuticle of the sides in contact was as perfect as usual. 



What, then, is the power which the Saxicava and its con- 

 geners employ ? The question, Mr. Osier admits, has not 

 been perfectly resolved : but it is probably an acid excreted 

 by the animals, capable of softening or dissolving lime. An 

 objection to this may be taken from some facts already 

 mentioned, viz., that these shellfish are sometimes found in 

 argillaceous as well as in calcareous rocks. The facts, how- 

 ever, admit of explanation ; for the young animals may be 

 supposed to fix themselves in holes or crevices convenient 

 for their purpose, and which afford them immediate shelter. 

 Hence they are occasionally found lodged among the en- 

 tangled roots of sea-weed ; and they will sometimes find a 

 shelter in rocks upon which they are unable to act chemically. 

 And that this explanation is correct may be proved by the 

 examination of the cells, which are not smoothed and fa- 

 shioned to the shape, as they are when excavated in lime- 

 stone : and, indeed, when burrowing in the latter, if the 

 animal meets with a piece of clay or feldspar, its progress is 

 immediately stopped, or the shape of the shell is deformed 

 by the pressure of this insoluble substance. The cells in the 

 pillars of the temple of Jupiter Serapis afford examples of 



