408 MB. J. C. HAWKSHAW ON THE 



limpets in the course of a year ; but they must repeat the abra- 

 ding process many times if they can, as some do, confine their 

 operations to a few square inches of surface. Some of the best- 

 defined grooves which I measured were -^q of an inch in depth ; 

 but I think that the limpets in grazing over a surface which has 

 been previously grooved have a tendency to deepen the first-made 

 grooves in the centre ; and if so, the above depth might be the 

 result of several operations. As nearly as I can estimate it, the 

 depth of chalk removed on a fresh surface is about '006 of an 

 inch ; so that if we suppose the limpets to feed over the same area 

 of surface ten times in a year, the total depth of chalk removed- 

 will be "06, or about j'y of an inch. In any case they do more to 

 destroy the rock-surface than the sea ordinarily does. If this 

 were not the ease, the action of the sea would obliterate the 

 marks made by the limpets, which it does not ; for the surface of 

 the chalk is free from the marks or grooves only along the base of 

 the clifi's where the shingle is washed about by the waves, and in 

 a few holes and gullies where loose pebbles are rolled to and fro. 

 The limpets do a great deal of apparently unnecessary work in 

 rasping away so much chalk ; but it may be beneficial to them in 

 preventing the settlement of sedentary rivals, such as JBalani or 

 the larger seaweeds, and so enabling them to keep a large sur- 

 face of pasture-ground to themselves. The rasped surface seems 

 to be soon covered again by the fine green coating on which, I 

 presume, they feed. They rasp close round any hard object, 

 such as a piece of shell or flint imbedded in the chalk ; so that 

 any Balanus or other sedentary growth would be left on an exposed 

 pedestal of chalk, and, as the chalk is soft on the surface, would 

 be liable to be washed off by the waves. On a large block of 

 chalk which was tenanted by a quantity of limpets, so that every 

 part of the surface was rasped over by them, I noticed one or 

 two solitary Balani. The raspings extended close round the base 

 of the shells of the Balani, and must have tended to weaken their 

 hold on the rock. Tet a large proportion of the shells of the 

 limpets had five or six large Balani on them. It would appear 

 probable from this that there was something which made the 

 chalk an unsuitable resting-place for Balani ; and the action of the 

 limpets may not unlikely be the cause. The limpets certainly 

 had the foreshore almost entirely to themselves down to low- 

 water mark. These comparatively large areas of rock-surface 

 covered only by a short vegetable growth, and browsed over by 



