410 



ON THE ACTION OF LIMPETS ON CHALK. 



would be to diminish the internal capacity of the shell, and pos- 

 sibly to cause discomfort to the animal, or prevent its obtaining 

 a firm hold on the rock. As all the surface of the chalk 

 outside the shell becomes covered with the fine growth of sea- 

 weed, the outer side of the groove round the edge of the shell, 

 which forms the side of the pit, becomes in like manner covered 

 with seaweed, and is pared away to a slope. This assists the 

 cutting eff'ect of the edge of the shell, as it is more effective 

 against the foot of a slope than it would be if the face of the pit 

 were perpendicular. I noticed one case in which a limpet ap- 

 peared to have pared away one side of the pit, that opposite the 

 head of the animal, as fast as the pit had been sunk. The animal 

 had begun to browse from the edge of the shell outwards. 



The above appears to me to be an explanation of the manner 

 in which the habit of sinking pits may have been acquired by 

 limpets. But in many cases they now appear to excavate deeper 

 pits than would be required for the removal of the protuberance, 

 extending the excavation below the plane of the rim of the shell. 

 For what purpose this is done I do not know, unless it be to get 

 a clean surface of chalk to adhere to, as their slimy bodies would 

 detach pieces of chalk in time, and possibly render their hold 

 less secure. Small pieces of chalk do adhere to the animals when 

 you remove them from the rock. These hollows which they ex- 

 cavate below the plane of the rim of the shell are, when com- 

 pleted, basin-shaped, sloping away from the edge of the shell. At 

 first they are begun beneath the head of the animal, and a consi- 

 derable hollow is often made there before the excavation is ex- 

 tended round the sides backwards. During the process of exca- 

 vation a lump is left in one stage in the centre. 



"When a limpet has sunk some distance into the chalk by the 

 above processes combined, the pits are further enlarged by smaller 

 limpets sinking secondary ones and browsing on the seaweed 

 which grows on the sides of the pits. 



I noticed signs that limpets prefer a hard smooth surface to a 

 pit in the chalk. On one face of a large block, over all sides of 

 which limpets were regularly and plentifully distributed, there 

 were two flat fragments of a fossil shell about 3 inches by. 4 inches, 

 each ^imbedded in the chalk. The chalk all round these frag-' 

 ments was free from limpets ; but on the smooth surface of the 

 pieces of shell they were packed as closely as they could be. I 

 noticed another case which almost amounts, to my mind, to a 

 proof that they prefer a smooth surface to a hole. A limpet had 



