FAMILY 2. PHOLADARIA. 41 



within or on any part that requires protection, according to the neces- 

 sities of the animal. They are therefore mostly found over the hinge, as 

 it is still a matter of doubt whether the Pholas forms any permanent 

 ligament. 



From the circumstance of the Pholades being found to inhabit the 

 hardest descriptions of calcareous rocks, we are led to suppose that the 

 cavities in which they dwell are formed by the aid of some powerfully 

 solvent secretion, operating with the constant current of water around 

 the shell, as the fine striae on its surface disprove that there is any rota- 

 tory motion. This current, which is necessary to the existence of the 

 animal, is said to be produced by the incessant motion of minute vibra- 

 tile cilia, which cover not only the whole of the branchiae, but other parts 

 of the body * . 



Examples. 



PL XXIII. Fig. 1 and 2. 



Pholas costata, Linnaeus, Syst. Nat., p. 1111. Lamarck, Anim. sans 

 vert., new edit., vol. vi. p. 45. Enc. Meth., pi. 169. f. 1, 2. 



PL XXIV. Fig. 1. 



Pholas dactylus, Linnaeus, Syst. Nat. p. 1110. Lamarck, Anim. sans 

 vert., new edit, vol. vi. p. 43. Enc. Me'th., pi. 168. f. 4. 



PI. XXIV. Fig. 2. 

 Pholas striata, Sowerby, Genera of Shells, No. 23. 



PL XXIV. Fig. 3. 

 Pholas papyracea, Sowerby, Genera of Shells, No. 23. 



* For further observations on the terebrating property of the Mollusca, see note on Saxi- 

 cava, page 71. 



VOL. I. 



