iiltim ate particles. This operation is by no means extra- 

 ordinary, as every anatomist is aware that the bony por- 

 tions of the animal frame are universally modified by the 

 action of the softer parts. In many of the Annelides we 

 find animals of a very soft, almost gelatinous structure, 

 penetrating the hardest calcareous rocks, and into the sub- 

 stance of the thick valves of many shells. These analo- 

 gies lead us to the conclusion that the Lithophaga exca- 

 vate a lodgement in solid substances not by the friction or 

 boring of their shells, but by the operation of their soft 

 parts upon them, and not, as a distinguished naturalist has 

 recently supposed, exclusively by maceration of their an- 

 imal mucus. There are however some facts which seem 

 to indicate the presence of a solvent. Mr. Osier has a 

 specimen of a hard calcareous rock in which small masses 

 of silex remain in relief on the sides of excavations formed 

 by Saxicava rugosa and Vcncrupis irus ; and another spe- 

 cimen of lime mixed with argil, in which the progress of 

 three Saxicavas was arrested by a thin layer purely argila- 

 ceous. 



PETHICOLA PHOLADIFORMIS. 



DLSCltlFriOX. 



Shell transversely elongated, white : poslcrior sidr vcrv 

 short ; anterior side a little gaping : hinge and dorsal nuir_ 

 g-ins nenrly parallel: surface longitudinally radiated wit!i 

 elevated lines, which, anterior to the niiddlearc but slightly 



PL 60. 



