LITHOPHAGA. 343 



sliells, but in the absence of any tube or accessory pieces. 

 It includes only two genera — Saxicava and Petricola. 



The shell of the first genus presents such a remarkable 

 diversity of character at different periods of its growth, as 

 to occasion the same species to be assigned to different 

 genera. Thus the well-known Saxicava rugosa, when 

 covered with small spines, has received a second name, Hia- 

 tella arctica ; and when still younger, a third, Solen minutus ; 

 and not till the spines were lost, the teeth became obsolete, 

 and the shell had assumed an irregular growth from its 

 habit of terebrating, has it been recognized. 



This curious mollusk is a most skilful borer of limestone 

 rocks, and his operations have been carried on during such 

 long periods as to destroy large masses, and make deep 

 water where shoals previously existed. Hence the huge 

 blocks of Portland stone, to which the buoys were formerly 

 attached, were punctured in the course of two or three years 

 on the surface, and also deeply perforated by this indefati- 

 gable miner. The sea-walls of Devonport Dockyard, which 

 are formed of Portland stone, are likewise honeycombed and 

 frittered away, below the low- water level of spring tides, by 

 the same unceasing agency. Along the base of the sea- 

 walls may be found innumerable SaxicavcBj nested in their 



