PHOLADID^. 93 



epidermis : it does not show the slightest indication of 

 any corrosive action ; and the inside is highly polished, 

 although in close contact with the mantle. 



It is the Chama parva of Da Costa_, P/iolas pusilla of 

 Poli and Olivi (but not of Linne) , P. faba of Pulteney, 

 Mya Pholadia of Montagu, P. Mans of Brocchi (but not 

 of Chemnitz), G. modiolina of Lamarck, Mytilus ambi- 

 guus of Dillwyn, G. pelagica of Risso, G. cuneiformis of 

 Philippi (but not of Lamarck), as well as his G. Polii 

 and G. Poliana, G.fulva of Leach, and G. tarentina of 

 Costa. 



Family XXIII. PHOLA'DID^, Gray. 



Body conico-cylindrical : mantle thickened at its outer edges, 

 and reflected behind, where it covers the hinge of the shell : 

 tubes large, extensile ; orifices of both or of one of the tubes more 

 or less cirrous : gills, a pair on each side, narrow, for the most 

 part adherent on one of their sides, and prolonged into the 

 branchial tube : palps also two on each side, coarsely pectinated 

 as well as the gills: foot short and sucker-like, never bys- 

 siferous. 



Shell wedge-shaped, convex, equivalve, inequilateral, widely 

 gaping in front (except in the adult Plioladidea and allied 

 genera, which have the gape closed by a shelly layer), and at 

 the posterior end in all the genera but Xyloptliaga : epidermis 

 membranous, thin : heal:s not prominent : hinge connected by 

 the anterior adductor muscle, which supplies the place of a 

 ligament; it is covered by a thickened fold of the mantle, 

 which is to a greater or less extent protected externally by 

 one or more testaceous shields or plates ; the hinge is in ar- 

 ticulated, but sometimes furnished with laminar or tubercidar 

 processes : apophyses, as in Terehratida, falciform, springing 

 forwards from beneath the hinge, one in each valve : pallial 

 and muscular scars indistmct. 



These burrow in stone, clay, mud, sand, wood, peat, 

 and other mineral and vegetable substances. In the 

 holes thus excavated they dwell at ease, never of their 



