COXCniFEEA. 



i05 



Tlie common i)i(ldock is used, for bait on the Devon coast ; it3 

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

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

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

 food. 



P. dadylus has two accessory yalves to protect the umbonal 

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



Fig. 269. Pholas d-.'ctyhis. Chalk, Sussex Coast. 

 u, 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 inirva have a single umbonal shield, 

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

 In P. crispata,Jj. [Zirfcm^ 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 



Distrihution, 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. 



PnoLADiDEA, Turton, 1819. 



Ti/2->e, P. papyracea, PI. XXIII. , Pig. 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. 



Distrihution, 7 species. ' Britain, New Zealand, Ecuador. 

 Low tides — 10 fathoms. 



Suh-gcnera, Martesia (Leach), BL 1825. M. striata, 

 PL XXIIL, Fig. 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 



