48 GENUS III. PHOLAS. 



enable it to bore the holes in which it is found ; but most pro- 

 bably it first softens the substance by means of some fluid 

 which it may have the power of secreting from its own body : 

 several species are known to be phosphoric, shining with 

 great brilliancy in the dark. They are found lodged in holes, 

 which they perforate in the rocks, or in wood, or clay, &c. 

 The outer orifice is small, by which they enter when young, 

 enlarging the internal cavity as they increase in size, so as 

 completely to imprison themselves, but always to such a depth 

 that they can reach the movith of their cells with their tubes 

 Avhen they extend them to take their prey. 



Linnaeus has described 6 species, Gmelin has added 6, and 

 Dr. Turton none : making a total of 13. 



Lamarck has made no alteration in this genus ; he describes 

 the Pholas as a transverse gaping shell, composed of two large 

 principal valves, with several small accessory pieces placed on 

 the cartilage or hinge (see Pholas Dactylus Linn. pi. 1. f. 7, 

 8, and 9). 



Those marks or ridges on the shell that are nearly parallel 

 to the outer margin (as in p^. L /. 7) are said to be transverse, 

 and those which are in lines radiating from the hinge to the 

 margin are called longitudinal (^pl. L /. 7). 



