COIs^CHIFEEA. 



505 



The common piddock is used for bait on the Devon coast ; it? 

 foot is white and translucent when fresh., like a piece of ice ; the 

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

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

 food. 



P. dactylus has two accessory valves to protect the umbonal 

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



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

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



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

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

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

 In P. crispatct, L. {Zirfcea, Leach), the umbonal shield is not dis- 

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

 of the vflves is divided into two areas by a transverse fiiiTow 



Dist'Abution, 32 species. United States, Norway, Britain, 

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

 Zealand, "Western America : — 25 fathoms. 



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

 Europe. The secondary species belong to the next group. 



Pholadldea, Turton, 1819. 



Type, P. papyi-acea, PI. 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 homy cup at their base. 



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

 Low tides — 10 fathoms. 



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

 PI. XXin., Pig. 21. Yalves 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 



