MANUAL OF THE MOLLUSCA. 



has frequently a calcarious lining, within which the shell remains free; 

 Teredina cements its valves to this tube when full-grown. The opening of 

 the burrow, at first very minute, may become enlarged progressively by the 

 friction of the siphons, whieh are furnished with a rough epithelium ; but it 

 usually widens with much more rapidity by the ivastiny of the surface. As 

 the timber decomposes the shelly tubes of the Teredo project, and as the 

 beach wears away the pholas burrows deeper. 



PHOLAS, L. Piddock. 



Etym. Pholas, a burrowing shell-fish, from pholeo, to bore. 



Type, P. dactylus, fig. 222. Ex. P. Bakeri, PI. XXIII. fig. 19. 



Shell elongated, cylindrical ; dorsal margin protected by accessory valves ; 

 pallial sinus reaching the centre of the shell. 



Animal with a large truncated foot, filling the pedal opening ; body with 

 a fin-like termination; combined siphons large, cylindrical, with fringed 

 orifices. 



The common piddock is used for bait on the Devon coast ; its foot is white 

 and translucent when fresh, like a piece of ice ; the hyaline stylet (p. 29) lodged 

 in it, is large and curious. P. costata is sold in the market of Havannah, 

 where it is an article of food. 



Fig . 22. Pkolas dactyhis. Chalk, Sussex Coast. 

 u, umbonal valves ; p, post-umbonal valve ; d, dorsal valve. 



P. dactylus has two accessory valves to protect the umbonal muscle, 

 with a small transverse plate behind ; a long unsymmetrical plate fills up the 

 space between the valves in the dorsal region. P. Candida mA parva have a 

 single umbonal shield, and no dorsal plate; these differences are only of 

 specific value. In P. crispata, L. (Zirfaa, Leach) the umbonal shield is not 

 distinctly calcified, but there is a small posterior plate ; the surface of the 

 valves is divided into two areas by a transverse furrow. 



Distr. 25 sp. U. S. Norway, Brit. W. Africa, Medit. Crimea, India, 

 Australia, N. Zealand, W. America : 25 fms, 



Fossil, 25 sp. (U. Lias ) Eocene , U. States, Europe, The secondary 

 species belong to the next group. 



