PHOLAS, 23 



lined by layers of the decomposed but reunited rock in which 

 it is bored. These varieties in structure, all with one ob- 

 ject evidently in view, rendered my task in preparing figures 

 and descriptions of all the species of Pholas one of no ordi- 

 nary pleasure. These animals are said to have the remarkable 

 property of throwing off a phosphorescent light, of a bluish- 

 white hue, which is believed to proceed from a luminous 

 mucus like that of the Medusa. It is given out into the 

 surrounding currents, so that -we have the phenomenon of a 

 stream of hght issuing from a hermit^s darkened cell. 



We have not met with any persons who have tasted our 

 British PJiolades ; but the P/iolas costata, a West Indian 

 species of some size, is much coveted as food, and sold in 

 the markets for the purpose. 



The species of British Pholas are as follows : — 



1. P. dactylus: the large long kind, which has two long and 



one cross accessory valves on the hinge, and one long 

 and narrow between the edges below. It is com- 

 mon on the southern coasts of England. Plate II. 

 fig. 1, 2. 



2. P. crispata : with very short, broad, wide and gaping 



valves, and scarcely any dorsal plate. It is often pre- 

 served with a coriaceous covering for the canals. 



