98 PHOLADIDiE. 



in Lithodomiis ;) which would be rubbed off during the 

 operation of boring ; 4th. because, though some borers 

 have spiny shells, others have smooth ones : 5th. because 

 all bore into calcareous substances, wood excepted ; and 

 into sandstone only when it has lain a long time under the 

 sea, and become as soft as clay. These objections of Mr. 

 Gray to the mechanical theory arc some good, some bad ; 

 several not consistent with a correct knowledge of the habits 

 and structure of the genera he quotes. The same natural- 

 ist, in a paper on the structure of Pholades, in the 1st 

 volume of the "Zoological Journal," 1825, held an oppo- 

 site view to that quoted above ; for he here maintains that 

 the Pholades bore by means of rasping. Dr. Fleming's 

 most recently-expressed opinion on this subject,* is in 

 favour of rasping and rotatory motion. 



Among the best memoirs on the subject of the boring 

 mollusks. Is that by Mr. Osier, published In the " Philoso- 

 phical Transactions" for 1826.t It is entitled " On Bur- 

 rowing and Boring Marine Animals,"" and contains the 

 fruit of much careful and original observation. According 

 to this gentleman, the instinct to bore is exhibited at a very 

 early stage of the anlmaPs life. He found Pholades com- 

 pletely buried when so minute as to be almost invisible. 

 He regards the curved processes, or apophyses, within the 

 shell of Pholas, as characteristic of an animal which bores 

 mechanically by employing Its shell as a rasp, holding that 

 the shell is the chief instrument by means of which Pholas 

 bores. He remarks with respect to Pholas candidus, a 

 species whose habits he observed with great care, that it Is 

 by means of the anterior and lower part of the shell, which 

 is thicker and spiny, the boring is effected. He considers 

 the peculiar arrangement of the muscles, and the suppres- 



* Molluscous Animals, p. 114. f Vol. cxvii. p. 356. 



