868 MR. E. S. IIUSSELL ENVIRONMENTAL [NoV. 26, 



The difference between rough shells and smooth shells must be 

 directly due to the difference of the rock surfaces on which they 

 occur, for this is the sole difference between their respective 

 environments. Smooth limpets and rough limpets (on different 

 stones, of coiu'se) may be found within an inch or two of one 

 anothei", and so subjected to environmental influences identical 

 except in one respect. Moreover, since limpets do not wander 

 from stone to stone (Section I.), the difference in environment 

 between rough and smooth shells is constant during their lives. 

 The differences between the two types of shell are therefore caused 

 by the diffei-ence in a single environmental factor. 



The question ai'ises, in what way does the surface of the rock 

 modify the growth of the shell so as to give rise to two distinct 

 types of shell, according as the surface is rough or smooth ? It 

 is necessary first to state in what particulars the two shells 

 differ. 



Differences betioeen the two types. — As stated above, the smooth 

 type corresponds more or less to the variety ccerulea L. (the 

 P. ccerulea of Linne, S. N. p. 1259), which is thus defined by 

 Jeffreys : " Shell depressed, roundish oval, ribs more delicate and 

 less regular, inside dark blue." It is found " on flat stones and 

 slabs of rock at low water." 



The chief peculiarities of the smooth shell are its regular 

 outline, and the number and minuteness of its ribs (text-figs. 223, 

 225, & 227). When the shell is small, say below 20 mm., there 

 are a large number of fine ribs, 12-14 of these being slightly more 

 distinct than the others. When the shells grow bigger these 

 12-14 ribs cease to be at all prominent, and the surface becomes 

 covered by small fine ribs, all more or less equal in size. In 

 rough shells, on the other hand, the outline of the rim is irregular, 

 and some of the ribs are much more prominent than the rest (text- 

 figs. 224, 226, & 228). Rough shells of 9-15 mm. in length have 

 only 12-14 ribs. Shells of larger size have these 12-14 ribs, and 

 usually a few more, very prominent, together with minute ribs in 

 between the principal ribs. 



Rough shells are usually heavier and thicker than smooth 

 shells, and the thickening of the rim takes place earlier in 

 them. 



The differences between the two types are most evident in 

 medium-sized shells and tend to become obscured in large shells 

 by the efiects of weathering and erosion. Small specimens of the 

 smooth tj^pe sometimes have their shells gaily coloured with little 

 radiating streaks of white and red and blue, but the geneiul 

 colour of large smooth shells is dark green or brown. Rough 

 shells are coloured in various shades of brown and grey, the ribs 

 being usually tinged with reddish brown. 



Effect of Roch Surfcice on character of Shell. — It is easy to see 

 in a general way that a lim23et growing on a smooth surface will 



