CONCHIFERA. 505 
‘ The common piddock is used for bait on the Devon coast; its 
footis 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- 
Dy 
See 
a4 
a ta Aeneas Ss ae k 
Rn Se SE 
BARBS SS > 
eS te 
enon SSO =< 
WORE A 
Fig. 269. Pholas dactylus. Chalk, Sussex Coast. 
u, umbonal valves; y, post-umbonal valve; d, dorsal valve. 
metrical plate fills up the space between the valves in the dorsal 
region. LP. candida and parva haye a single umbonal shield, 
and no dorsal plate; these differences are only of specific value. 
In P. crispata, L. (Zirfoea, Leach), the umbonal shield is not dis- 
tinctly calcified, but there is a small posterior plate; the surface 
of the yelyes is divided into two areas by a transverse furrow 
Distribution, 32 species. United States, Norway, Britain, 
Western Africa, Mediterranean, Crimea, India, Australia, New 
Zealand, Western America :—25 fathoms. 
Fossil, 25 species. (U. Lias—) Hocene—. United States, 
Europe. The secondary species belong to the next group. 
PHOLADIDEA, Turton, 1819. 
Type, 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. 
Distribution, 7 species. Britain, New Zealand, Hcuador, 
Low tides—10 fathoms. 
Sub-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 valyes. 11 species. 
West Indies, Africa, India. MV. striata burrows in hard timber. 
Z 
