440 BORING MOLLUSCA. 



Many of the boring Mollusca, especially the Lithodomi 

 and Petricolae, cover the hinder part of their shells with a 

 calcareous coat, which is often of a spongy texture, and 

 differs from the shell in internal structure. This is proba- 

 bly the dissolved part of the rock again deposited. Many 

 also of these animals, as the Gastrochaense, Clavagellae, and 

 Teredines, secrete constantly, and others, as the Lithodomi, 

 under particular circumstances, a calcareous deposition, with 

 which they line the inner surface of their holes. 



The determination of the existence of this power of dis- 

 solving shell and calcareous matter does not, however, remove 

 the difficulty with regard to those shells which bore into 

 wood ; although it is not impossible that this substance may 

 also be dissolved by the same means. And this appears to 

 me the more probable, as, although there are some species of 

 Pholades, such as Pholas pusillus and P. rudis, which I have 

 never seen in any other substance, I have found others, such 

 as P. dactylus and P. candidus, indiscriminately in chalk, 

 marl, limestone, red sandstone, and wood ; and it is difficult 

 to suppose that these species adopt different means of boring 

 when employed in penetrating the latter substance. 



Possessing this power of absorbing their own shells, the 

 shells of other Mollusca, and calcareous rocks, it is remark- 

 able that these animals do not exert it for the purpose of 

 removing extraneous obstacles which may oppose their pro- 

 gress in the formation of their shells. In the collection in 

 the British Museum is a specimen of Pyrula bezoar which 

 appears to have grown with perfect regularity until the for- 

 mation of its last half-whorl, which is thrown considerably 

 more than half an inch out of its proper position by a group 

 of Barnacles. These shells had probably attached them- 

 selves to the back of the Pyrula at an earlier stage, and, as 

 the latter increased in size, at length filled the place that 

 should have been occupied by the inner lip, which, on meet- 

 ing with this interruption, diverged from its course, and was 

 thrown over the Barnacles. Had the shell not been taken 

 until a later period, there can be little doubt that the animal 

 would have at length destroyed the Barnacles, and completely 

 hidden them from view, by continuing the whorl entirely 

 over them ; although it would appear that it had not the 

 power to remove them by absorption while they retained 

 their vitality. In the same collection there is also a speci- 

 men of S trombus luhuanus the spire of which has been 

 much distorted in consequence of the temporary attachment 

 of some parasitic shell, which subsequently became loose and 

 has been detached. 



