BORING MOLLUSCA. 439 



ostracuui, which is thin in the Teredines, Pholades, Laseas, 

 &c., thick in the Lithodonii, and which, if the animals used 

 the outer surface of the shell as a means of boring, must be 

 very speedily rubbed off. Such a fact would be readily 

 observed, as this part is never renewed after having once 

 been destroyed ; which is easily understood when we con- 

 sider, that it can only be formed on the edge of the shell 

 before the deposition of the shelly matter has advanced 

 beyond it. 



4thly. That although the shells of Teredines, Pholades, 

 some Petricolae, &c., are covered with short spines and strias 

 by means of which they might be supposed capable of rasp- 

 ing stones, other boring shells, such as the Laseae and Litho- 

 phagi, are smooth. 



5thly. That I have not observed shells of this kind to bore 

 into any other substances (wood excepted,) than shells, cal- 

 careous rocks, clay, marl, chalk, limestone, and sandstone 

 united by a calcareous cement ; nor do such shells, as far as 

 I have seen on the coast of Devon, attack the latter rock, 

 except when it has lain a long time under the sea, and be- 

 come as soft as clay. Colonel Montagu states that he has 

 seen specimens of Gastrochaena which had bored into fluor- 

 spar and granite ; but an examination of his specimens in 

 the British Museum proves that what he considered as fluor- 

 spar are merely crystals of carbonate of lime ; and although 

 the shell is not uncommon on the coast of Cornwall and 

 Guernsey, I have never seen it produce the slightest impres- 

 sion on the granite rock, even in its disintegrated state. 

 Instead of attempting this, the animal changes its habits, 

 and generally chooses a slight crack in the granite rock, in 

 which it forms for itself, like some of the fossil species of the 

 genus called Fistulana by Lamarck, a calcareous case, partly 

 constructed of such fragments of shells or stones as may be 

 thrown within its reach. The granite, indeed, appears com- 

 pletely to resist all the dissolving powers of the Mollusca. 

 Thus in some structures, as the Plymouth Breakwater, for 

 instance, in which limestone and granite are employed toge- 

 ther, and placed side by side, the Patellae form their rounded 

 holes in the former ; while they do not in the shghtest de- 

 gree alter the surface of the latter, except in general by 

 clearing off any calcareous substance which may have pre- 

 viously grown upon it. I have one specimen beautifully 

 illustrating this latter fact, a young Patella having ailixed 

 itself to the shelly base remaining from a Barnacle, in which 

 it has dissolved only the part beneath its foot, leaving the 

 rest forming a ring around its shell. 



