236 Mr. A. Hancock on the Boring of the Mollusca into Rocks, 



these lines might, in some way, be connected with the structure 

 of the shell, though it was impossible to explain how this could 

 be. Since then I have procured from the coast, in the neigh- 

 bourhood of the Tyne, specimens of the bores of the common 

 Patella in shale and limestone very distinctly marked with the 

 same radiating strise or scratches; thus proving satisfactorily 

 that the radiating lines in the former instances are not structural, 

 and that the foot and mantle of the animal possess the power of 

 reducing those substances mechanically. But certainly not by 

 ciliary currents, unless they be supposed capable of producing a 

 scratched surface ; and if so, how are the currents in the cases 

 cited to pass away, meeting as they must under the centre of the 

 foot* ? 



Having stated the facts, and having gone through the reasons 

 that induce me to consider the three generally received theories 

 insufficient to account for the operations of the stone- and wood- 

 boring mollusks, I shall now proceed to explain my own views 

 on the subject. 



It has been already stated that I have expressed an opinion 

 that the anterior portion of the animal is the excavating instru- 

 ment. This in Teredo and Pholas is composed of the foot and 

 the edges of the mantle, which, together, entirely fill up the 

 frontal gape of the shell. In Sawicava and Gastroch&na it is 

 formed wholly of the edges of the mantle, which are united and 

 thickened. The foot and mantle of Teredo protrude before the 

 valves : the former is circular and convex in front, and there is 

 little doubt, from the resemblance it bears to the same part in 

 Pholas, that it adheres to the bottom of the burrow, with which in 

 form it precisely agrees. Sir Everard Home calls this the " pro- 

 boscis," which, he says, " in the living animal had a vermicular 

 motion : the extremity was covered by a cuticle not unlike the 

 cornea of the eye." Sir Everard also remarks, " that as this 

 proboscis has no orifice, there is reason to believe that it adheres 

 to the wood, acting as a centre-bit, while the animal is at work 

 with the shell." Not having seen Teredo alive, I cannot speak 

 from my own knowledge to the exact form of these parts ; but 

 from the examination of specimens in spirits I have no doubt of 

 the accuracy of this description, which proves that the foot or 

 " proboscis n corresponds exactly in form to the bottom of the 

 burrow. The comparison of this part to the " cornea of the eye u 



* In the ' History of British Mollusca/ it is stated that in Mr. Cuming's 

 collection there is a specimen of Pholas which has made its excavation in 

 wax ; a circumstance scarcely favourable to the opinion advanced by the 

 authors of that work. Currents of water could have very little effect on a 

 substance of this nature, and it would be very liable to clog the rasping sur- 

 face of the valves so as to retard operations, if not to stay them entirely. 



