236 LIFE BY THE SEASHORE. 



of darker tint, which run in the opposite direction to the 

 lines of growth. Shells found on the beach often have the 

 outer coat rubbed off, and are then "silvery," that is, they 

 show the mother-of-pearl lining. 



The large Trochus zizyphinus (see Fig. 69) is on the 

 east an inhabitant of deep water; on the west it occurs 

 between tide-marks, and the shell is common on most 

 shores. The shell is conical in shape, and may reach a 

 height of over an inch, the base is without a perforation, 

 there are eight or ten whorls, and the shell is usually 

 spotted with bright rose colour. In the general case the 

 species is readily recognised. 



There are very many other species of Trochus found in 

 Britain, some in deep water, and some found in the tidal 

 pools in the South and West. 



The next set of forms we shall consider are the peri- 

 winkles, an interesting and puzzling group. As the name 

 of the genus Littorina indicates, they are purely littoral 

 forms, living almost exclusively between tide-marks, and 

 showing much tolerance of fresh water and of dryness. On 

 the one hand they are related to the genus Lacuna, whose 

 members inhabit the Laminarian zone and deeper water, 

 and on the other they are connected with Paludina, a genus 

 of fresh -water forms, and they occupy every variety of 

 habitat between those of these genera. High up on the 

 cliffs, out of reach of all but the spray, on the stones of 

 the streams which run down the beach, on the tidal rocks, 

 on the broad blades of Laminaria, there are few localities 

 on the shore in which the ubiquitous periwinkles do not 

 occur. It is true that in most cases the different localities 

 are characterised by one dominant form, but in not a few 

 cases the species themselves have a wide range, and I have 

 picked four so-called species off one stone. You probably 

 do not need to be told that this is very exceptional, and for 

 a very obvious reason. It is an axiom of the modern theory 

 of evolution that those divergences of structure which ulti- 

 mately result in the formation of new species have been 

 produced by divergences in the environment. Take as an 

 example the two species of porcelain -crab already con- 

 sidered (p. 179). The minute porcelain -crab is adapted to 

 one environment, the hairy porcelain -crab to another, and 



