1907.J STUDIES ON THE LIMPET. 865 



on exposed flat siu-faces are tjqjically low and broad, and that 

 shells on sheltered sm-faces are typically high and narrow. 



The results obtained by Cooke (2) for shells of Purpu.ra lapillus 

 may be recalled here. He found that shells in exposed places 

 were stunted and had a large mouth, and that shells in sheltered 

 spots were lai-ge and possessed a well-developed spire and a small 

 mouth. With regard to stunting, I find that a larger proportion, 

 namely 55 per cent., of the exposed shells are below 40 mm. in 

 length, than in the case of the sheltered shells, where the pi'o- 

 portion is 34 per cent. This fact may point to some stunting 

 among the exposed shells, but many exposed shells reach a great 

 size. 



Two other points of difierence between exposed and sheltered 

 limpets, and perhaps more important ones, become evident when 

 a number of exposed shells are directly compared with a number 

 of sheltered shells. Exposed shells are typically thick and hea^-y. 

 This thickness of the shell is j)robably a direct adaptation to the 

 greater shocks which an exposed shell has to withstand. The 

 second difierence is that exposed shells are much more irregular 

 in outline than sheltered shells. Of the shells which I kept on 

 account of their irregular outline, the majority came from exposed 

 limpets, and the outlines selected for illustration (text-figs. 217- 

 222) are all drawn from exposed shells. On the other hand, the 

 most beautiful symmetrical shells to be got are those which one 

 finds on the smooth under surface of a stone in a rock-pool, %. e. 

 in a very sheltered situation. It is natural that exposed limpets 

 should fit their shells to the irregularities of the rock to which 

 they cling, in oixier to gain support against the waves and tide, 

 and that (as will be shown in the next section) then- shells should 

 for this reason grow irregular in outline. 



To sum up, adult exposed shells of Patella are lower spired, 

 nai'rower, thicker, and more irregular in outline than sheltered 

 shells ; and these difierences are probably due to the diflference of 

 friction to which the two kinds of shells are exposed from waves 

 and tide. 



IV. Limpets on Rough Stones and Limpets on Smooth Stones. 



While I was collecting limpets for measurement at the 

 Southend of Arran I noticed that they were of two distinct types, 

 a " rough " type and a " smooth " type. The former was the 

 typical P. vulgata L., the latter the variety cairulea L. ( Jefii'eys, 9). 

 It became evident after a little careful observation that the rough 

 type occui-red always on rough stones, the smooth type always on 

 polished stones. 



I examined some 500 limpets with regard to their habitat, and 

 I found few exceptions to this rule. 



The detailed records are given in Tables "VTI., VIIL, and IX. 



