12 Mr. W. Clark on the Terebrating Mollusca, and on the 



in wood, as the exotic Pholas striata, the Teredines, and Xylo- 

 phaga dorsalis, the foot is the undoubted agent of perforation, 

 and in this class is probably armed with rasping additions, and 

 it cannot be doubted has the power to rub down the hardest 

 oak faster than the animal can require; in fact, the harder wood, 

 as oak, is more easily comminuted than the spongy deal or elm 

 plank. 



I take leave of this part of my present paper by again acknow- 

 ledging the great service Mr. Hancock has conferred on malaco- 

 logical science, by definitively, as I think, determining the true 

 functional causes of the terebrating powers in the Acephalous 

 Mollusca. 



I return to the question of the identity of the two forms of 

 Pholadidea papyracea. I have already shown that the great va- 

 riation in colour and markings between the adult Pholadidea 

 papyracea and the young shell styled Pholas lamellata is the 

 effect of generative influences, and that its conspicuous foot, when 

 it arrives at full growth, which is testified by its becoming com- 

 pletely domed, is depauperated and finally obliterated. These two 

 great and principal variations of aspect between the two forms 

 of Pholadidea papyracea, resulting from states of transition, ha- 

 ving I trust been satisfactorily disposed of, and every other part 

 of the animal exhibiting a prototype similarity, it is impossible, 

 as I think, to entertain further doubts of the positive identity of 

 the two shells usually termed by authors Pholas papyracea and 

 P. lamellata. I may add, that it has been asked in objection, 

 how is it that twenty Pholadidea papyracea are taken for one Pho- 

 las lamellata, and that the two forms are not more frequently 

 met with in the transition states ? This objection quickly yields 

 to a just view of the Pholades as regards habitat and other in- 

 fluences. 



The Pholades are usually inhabitants of the littoral zone, but 

 by no means always so, as some species also inhabit the more 

 pelagic zones ; the littoral shells are found in the superficial area 

 of the red sandstone rocks from half-tide to the lowest littoral 

 limits, and probably beyond, where they are unapproachable, at 

 the depth of a very few inches; the whole area of the lower por- 

 tion of the littoral zone is occupied by pele-mele colonies of Pho- 

 las parva, Saxicava rugosa and Pholadidea papyracea, generally 

 of adult proportions, with an intermixture of a comparative pau- 

 city of the form Pholas lamellata. The Pholas dactylus and P. 

 Candida usually inhabit the higher levels of the littoral zone. The 

 fact of the deficiency of the young of the Pholadidea papyracea 

 is occasioned solely by the pre- occupation of the area of the sand- 

 stone rocks by the species I have mentioned, mostly adult ; and 

 when the genial season of reproduction arrives, the fry are ejected, 



