PHOLAS. 99 



sion of the ligament in this genus, as peculiarities in its 

 organisation connected with its perforating hahits. Teredo^ 

 he holds, bores in like manner with Pholas, and by the 

 same means, effecting the stroke during the operation by 

 the contraction of the posterior adductor muscle. The 

 boring of Saxicava, however, Mr. Osier maintains to be 

 effected by very different means, most probably by an acid 

 solvent. Its hole is not round ; and if there are siliceous 

 particles in the stone they are left projecting into the 

 cavity. Mr. Osier was unable, nevertheless, to detect any 

 direct evidences of free acid, either by the test of litmus- 

 paper, or by any experiments he could devise. 



The account given by Mr. Osier of the operations of 

 Pholas when boring is so circumstantial, that we quote it 

 in his own words : — 



" The Pholas has two methods of boring. In the first, 

 it fixes itself by the foot, and raises itself almost perpendicu- 

 larly, thus pressing the operative part of the shell upon the 

 substance to which it adheres : it now proceeds to execute a 

 succession of partial rotatory motions, effected by the alter- 

 nate contraction of the lateral muscles, employing one 

 valve only, by turning on its side, and immediately regain- 

 ing the erect position. I have observed that this method 

 is almost exclusively employed by the very young animals; 

 and it certainly is peculiarly adapted for penetrating in a 

 direction nearly perpendicular, so that they may be com- 

 pletely buried in the shortest possible time. It may be 

 observed that the posterior extremities of the valves are 

 much less produced in the very minute PJiolades than they 

 afterwards become ; and thus the time required to complete 

 a habitation is still further diminished. 



" But when the Pholades have exceeded two, or at the 

 most three, lines in length, I have never observed them to 



