106 PHOLADID^. 



might possibly be found. . On the other hand, taking into 

 consideration its mineralogical nature, as stated by M. 

 Necker, there is no reason for supposing that the shell of 

 the PholadidcB is so weak a perforating instrument as some 

 have fancied. With its peculiar form, and the saw-like 

 asperities of its surface, especially of its anteal extremity, 

 it is well adapted for an auger, when wielded fresh and 

 elastic by its well-muscled animal inhabitant, whose foot in 

 all the members of this tribe, even in Teredo, where it is 

 least developed, seems specially organized to serve as a ful- 

 crum. We have no evidence that they perforate any sub- 

 stances essentially harder than their shells, or so hard. The 

 sandstones in which they occasionally occur are either friable 

 or marly when fresh, though cabinet specimens seem so solid. 

 The explanation of Necker accounts for their perforations 

 in the hardest limestones. Wood, wax, and other sub- 

 stances in which they occur, offer no difficulty. The state- 

 ments put forward respecting their boring in lava and gra- 

 nite have long ago been shewn to be mistakes. That they 

 exhibit a rotatory motion during the action of boring, has 

 been proved by competent observers ; and the cavities they 

 excavate, if examined when fresh, invariably show trans- 

 verse groovings, which could have been caused only by 

 such motions. Currents of water set in motion by cilia 

 doubtless aid materially the animal's operations, and possi- 

 bly may be the means by which the larvae effect their first 

 lodgement ; but, considering the arrangements of the parts 

 of the body in the adult animal, it seems to us that Mr. 

 Garner's view of their being the primary cause of the per- 

 foration, whilst the rasping of the valves is secondary, 

 should be reversed. Such currents must be most effective 

 in clearing away loosened and loosening particles. If there 

 be any chemical action aiding, it must be due to the carbo- 



