and on the removal of portions of their Shells. 235 



The under surface of the foot of Patella is clothed with vibra- 

 tile cilia ; but, as just stated, that part of the animal does not sink 

 so deep into the rock as the mantle, except when the surface 

 breaks up under the grasp of this powerful adhesive disc. There 

 are no cilia on the mantle, therefore the reverse of this might be 

 expected to take place were Mr. Garner's theory correct. The 

 cilia on the foot arc not at all peculiar to this animal : I have 

 detected them also in Doris, Limapontia, Purpura and Littorina ; 

 and it is worthy of remark that in all these they are very much 

 larger and more vigorous than in Patella, in which they are 

 so extremely small that it requires the greatest care and high 

 magnifying powers to make them out. 



Let us now for a moment suppose that these currents are ca- 

 pable of reducing the substances into which these animals pene- 

 trate, and that a Patella attached to shale, or any other rock, is 

 working its way into it. What are the currents effecting all the 

 time on the surface around the shell — those currents being ten 

 thousand times more powerful than the minute, microscopic ones 

 produced by the animal ? Must not the action of the advancing 

 and receding tides, augmented by the rolling of the waves to and 

 fro, triturate the surface of the rock more rapidly than the com- 

 paratively feeble, ciliary currents of the gills and foot ? Were 

 the excavating powers, therefore, limited to these currents, should 

 not the Patella stand upon an eminence rather than be half- 

 buried in a hole ? This argument also applies with equal force 

 to Pholas, and still more strongly to Teredo ; for surely it can 

 never be maintained that the triturating effect of water on the 

 planks of a vessel, sailing at the rate, occasionally, of ten knots 

 an hour, is less than the almost imperceptible currents produced 

 by the Teredo, which will penetrate several inches into the timber 

 during a voyage from India. 



I fortunately possess three or four specimens exhibiting the 

 bores of Patella, which perhaps may be considered sufficient to 

 prove that the animal works mechanically, and in a way that 

 currents of water could scarcely act. The first of these bores is 

 that of a small Patella : it is sunk only a little way into the sur- 

 face of a gigantic species of the same genus from the coast of 

 America, and exhibits minute strise or scratches radiating from 

 the centre to the circumference. I have seen another specimen 

 of this gigantic Patella, in the collection of Mr. Fryer, which is 

 marked with several similar impressions, one of which is scratched 

 in the same manner, but more distinctly. In this specimen the 

 whole surface of the impression is covered with lines radiating 

 from the extreme circumference to a spot near the centre. The 

 lines or scratches are in groups, and so small as to require a 

 powerful lens to show them. At first I thought it possible that 



