COXCHIFERA. 



505 



Tlie common piddock is used for bait on the Deyon coast ; its 

 foot is wMte and translucent when fresli, like a jnece of ice; the 

 hyaline stylet (p. 22) lodged in it, is large and curious. P. cosfata 

 i^ sold in the market of Havannah, -where it is an article of 

 food. 



P. dacti/lus has two accessory yalyes to protect the umbonal 

 muscle, with a small transverse plate behind ; a long unsym- 



Fig. 269. Pholas chdylus. Chalk, Sussex Coast, 

 w, umbonal valves ; j)-, post-iunboual valve ; d, dorsal valve. 



metrical plate fills up the space between the valves in the dorsal 

 region. P. Candida and loarva have a single umbonal shield, 

 and no dorsal plate ; these differences are only of specific value. 

 In P, crispata, L. [Zirfcea, Leach), the umbonal shield is not dis- 

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

 of the valves is divided into two areas by a transverse furrow 



JDistrihution, 32 species. United States, Norway, Britain, 

 Western Africa, Mediterranean, Crimea, India, Australia, New 

 Zealand, Western America : — 25 fathoms. 



Fossil, 25 species. (IJ. Lias — ) Eocene — . United States, 

 Europe. The secondary sj)ecies belong to the next group. 



Pholadidea, Turton, 1819. 



Ty2:)e, P. papyracea, PL XXIII. , Fig. 20. 



Shell globose-oblong, with a transverse furrow ; anterior gape 

 large, closed in the adult by a callous plate; 2 minute accessory 

 valves in front of the beaks. 



Animal with a.fringed disk at the end of the combined siphons, 

 and a horny cup at their base. 



Bistrihution, 7 species. Britain, New Zealand, Ecuador. 

 Low tides — 10 fathoms. 



Suh-genera, Martesia (Leach), Bl. 1825. M. striata, 

 PL XXIII. , Fig. 21. Valves lengthened behind, when full 

 grown, by a plain border ; umbonal valves 1 or 2 ; dorsal and 

 ventral margins often with narrow accessory valves. 11 species. 

 West Indies, Africa, India. M. striata burrows in hard timber. 



z 



