520 LECTURE XXI. 



expansion of the chamber had an irregular elliptical form in trans- 

 verse section ; and, moreover, one valve was fixed, so as to form the 

 lining-plate of its side of the chamber. The animal I dissected 

 {^Clavagella lata) had formed its chamber in a rock of calcareous 

 grit ; but a nearly allied species {Clav. australis) had bored its way 

 into siliceous grit ; whilst specimens of Clav. melitensis were en- 

 sconced in argillaceous tufa. A special solvent for each species 

 greatly complicates the chemical hypothesis. The mantle of the 

 Clavagclla presented, however, a peculiar modification, being ex- 

 panded into a thick convex cushion where it was applied to the 

 bottom of the chamber, through the development in its substance of 

 a mass of interlaced muscular fibres. The last-excavated part of the 

 chamber was, as it were, moulded to the surface of the cushion, 

 which was perforated by a minute slit for the occasional passage of 

 a filamentary foot. I suggested, therefore, this muscular develop- 

 ment of the mantle as being " one of the principal instruments in the 

 work of excavation."* But, viewing its attachment to the moveable 

 valve, and the strength of the adductor muscles, I supposed that that 

 valve might be applied, not only to effect the forcible expulsion of 

 the fluid from the pallial cavity, but also, " probably, to assist in the 

 excavation of the abode." Mr. Hancock f has recalled attention to 

 the excavating agency of soft and muscular masses in other boring 

 bivalves, analogous to that in Clavagella, as, e.g., the thickened 

 portion of the mantle in Saxicava and Gastroch(sna, and the foot in 

 Pholas and Teredo. 



If siliceous particles be actually secreted in the superficies of any 

 of the burrowing discs, they must add to their efficiency : and it is 

 certain that the perpetual renewal of a softer surface will render it 

 capable of wearing away a harder one, subject to the friction of such 

 softer surface, and not, like it, susceptible of being repaired. 



The admission of the wearing and boring power of muscular discs 

 need not, however, involve the rejection of the allied action of shelly 

 valves and ciliary currents. The diversity of the organisation of the 

 boring mollusks plainly speaks against any one single and uniform 

 borin2:-agent in all. 



The action of the foot and thickened border of the pedial aperture, 

 may be inferior to that of the valves in Teredo, as it certainly is 

 in Pholas. A valued correspondent, Mr. Robertson, of Brighton, 

 writes, " Between thirty and forty Pholades have been at work in 

 lumps of chalk, in a finger-glass and a pan of sea-water at my window 

 for the last three months. The Pholas dactylus makes its hole by 



* CCCXX. p. 271. t CCCXXII. 



