1907.] STUDIES ox THE LIMPET. 869 



tend to have a smooth shell-riiii ; for the rim of the shell is kept 

 closely aj^plied to the sui^face of the rock, and so the deposition of 

 shell-material along the rim is to some extent guided by the 

 surface, and must therefore be even. Similarly, if a shell is kept 

 applied in a definite way to a rough uneven surface, its rim must 

 in growth take on an irregular outline, and certain ribs be 

 emphasised while others are checked in their development. 



Now in many cases one can observe in rough limpets that the 

 shell is accurately adjusted to inequalities of the rock to which it 

 clings, or to barnacles on the rock, in such a way that the in- 

 dentations of the rim between the projecting ribs fit closely round 

 the projecting substRnces. Two or three cases of this adjustment 

 ai'e given above -(Section I.), and a few cases noted by other 

 observers may be adduced here. The phenomenon is in fact 

 quite common. 



Roberts (13) puts on record the following interesting observation 

 of a limpet which he watched returning to its " home." " I 

 watched his course : he arrived, and I immediately perceived a 

 difiiculty Avhich he made nothing of, viz., the getting adjusted ; 

 he slewed himself round, and fitted a little notch that was in the 

 edge of his shell to a small piece of projecting quartz with 

 wondei-ful readiness. He was tight in a moment, ready to resist 

 the heaviest breakers or any enemy." The limpet returned 

 daily to the same spot and adjusted himself. Hawkshaw (8) says : 

 " On the top of the smooth fractured surface of a pedestal of flint 

 a limpet had taken up its abode. The shell was closely adapted 

 to the uneven surface which it would only fit in one position." 

 Lukis and Lloyd Morgan record similar cases ; and this fitting of 

 the margin of the shell to the irregularities of the rock has been 

 observed also in Acmcea sjyectrum Reeve (Willcox, 15). 



An abnormal case which is significant for our purpose is given 

 by Dr. Robertson (14) : — " A case once came under my notice of 

 a> half-grown limpet that had got jammed between the inequalities 

 of two large stones. It obviously had been there for a considerable 

 time, as the shell had grown into a triangular shape to conform 

 to the walls of its prison." 



The irregular outline of the rim of the rough type of limpet- 

 shell is therefore probably a mechanical result of the accurate 

 adjustment of the rim of the shell to the inequalities of the rock 

 on which the limpet grows. 



Similarly, the regular outline of the smooth shell is due to the 

 rim being moulded during growth by the polished surface opposed 

 to it, and hence becoming smooth and regular. 



It is a well-known fact that Anomia takes the shape of the 

 surface to which it is adherent : if it grows on a Pecten shell it 

 bears the impress of the radiating ribs of the Pecten. Just in the 

 same way. Patella, since it is very sedentary and stays for months 

 adjusted in one definite way to the inequalities of the surface to 

 which it adheres, takes on at the edge of the shell the shape of 



