. BIVALVE MOLLUSCANS. 249 



to have no mechanical means of excavating its 

 crypt it can act solely upon the calcareous part 

 of the rocks it perforates for these and other 

 reasons, Mr. Osier is of the same opinion with 

 M. de Bellevue. 



Poli has described, a stone-boring bivalve, 

 belonging to the muscle genus, which perforates 

 marble, each inhabiting a separate crypt, ge- 

 nerally as large as the shell, and which he thinks 

 they enlarge by friction and rotatory motion. 

 The pillars of the temple of Serapis at Puteoli 

 were perforated by these animals at the height 

 of forty-six feet above the sea, whence it is pro- 

 bable they were so perforated before they were 

 carried there. 1 



When we compare the proceedings of these 

 four kinds of boring or burrowing Molluscans, 

 above described, with their forms, we shall find 

 in them a particular adaptation of means to an 

 end. In the ship- worm, whose province is to 

 penetrate into submerged timber and there to 

 take its abode, we find the anterior part of the 

 body armed with two shelly valves, moved by 

 strong muscles, which cut and rasp the sub- 

 stance upon which they act, so that it probably 

 begins its labour as soon as it is born, intro- 

 ducing its narrow body, defended at the other 

 extremity also by shell, into the timber softened 



1 Poli, ii. 215. 



