space, and in tlu.s, })erlia})s, lies llio need dI the de\el(»p- 

 meut of the homing- faculty, since a smooth surface is 

 required for fixation. Limpets cannot, moreover, success- 

 fully invade an area already occupied by these 

 competitors. Fixation is a protective measure against 

 the wash of the tide and the attacks of enemies, and the 

 normal adhesion can be largely increased when the shell 

 is touched. 



The limpet, with these habits, does noi seem to have 

 much to fear from active enemies when once the manifold 

 dangers of youth are past ; but it is to some extent eaten 

 by seabirds, by the common starfish, and even by rats, 

 while mau is, in some districts, a serious enemy. 



EXTEKNAL ClIAEACTEliS. 



I. SiiELL. — The shell is conical and its base has an 

 elliptical outline, the anteroposterior diameter being 

 greater than the transverse. The apex of the cone is in 

 the sagittal plane and nearer the front end ; its exact form 

 varies greatly, being sometimes, especially in young 

 limjjets, bent over and jiointing forwards. The external 

 markings of the shell vary remarkably within the limits 

 of the species, but there are always some lines radiating 

 from the apex and other lines concentric about that 

 point. The former are raised ribs running to the shell 

 margin, but they vaiy considerably in number and in 

 prominence ; the latter are lines of growth. The edge of 

 the shell is notched proportionately to the prominence of 

 the radiating ribs, and its rim is roughly chisel-shaped, 

 with the bevelled edge inside, and forming an efficient 

 instrument for enlarging the scar and deepening its 

 margin. There is considerable variation in the general 

 shape of limpet shells, those of animals living far up the 

 shore and on exposed flat surfaces being typically low and 



