120 chas. l. walton. 



The Littorinid^. 



It may be mentioned that the shells of L. ohtusata often bear a striking 

 resemblance in both form and colour to the vesicles and conceptacles of 

 the species of Fucus upon which they live ; so much so, in fact, that 

 when collecting this species, I have more than once mistaken the vesicles 

 of the Algce for the shell of the Mollusc. 



The conceptacles of all the species of Fucus are of shades varjdng from 

 white to bright yellow, and, indeed, the whole of the parts of the plant 

 exposed to sunlight are frequently much lighter in hue than the more 

 sheltered portions, which are usually dark brown or olive. One would 

 expect that the lighter coloiu'ed forms of L. ohtusata would tend to occur 

 upon the surface, and the darker among the masses of the plant. I have 

 made a considerable number of observations in the district with the 

 object of testing this idea. AMiat I have found is, that the darkest varieties 

 are seldom found upon the surface of the Algal masses, and that the 

 majority of varieties are found in abundance crawling on the exterior, 

 but are often in harmony with their surroundings. It may be mentioned 

 that many shells which appear out of harmony, are not so in reality, the 

 change of colour being due to drying of the shell during low water. The 

 greatest resemblance is between the light brown and yellow-shelled 

 varieties and the vesicles of F. vesiculosus and between the colour of the 

 frond and the ordinary brown and olive shells. Bright yellow Littorinas 

 may be found at times upon the dark stems of the Algae, and are then 

 very conspicuous. I do not find the yellow variety to be really common 

 in the district. 



The Littorinidse persist in the areas devoid of Trochidse ; and, as has 

 already been stated, L. ohtusata appears to be less affected by the adverse 

 conditions than the other species ; this is in all probability largely due 

 to the protection afforded it by the food plant, and its shell-characters. 

 L. rudis and L. littorea, are neither so large, so numerous, nor so evenly 

 distributed as in the areas where Trochidse are abundant. Several 

 varieties of L. rudis occur in Cardigan Bay, and of these tenehrosa, Montagu, 

 has so far only been found under stones some three miles south of Aber- 

 ystwyth on a portion of the shore influenced by fresh water. This in- 

 fluence was sufficiently strong to attract a number of eels which were also 

 hiding under the stones. Tryon describes the variety as " thin, whorls 

 roimded, with elevated spire, brownish or yellowish, usually tessellated 

 with white." In the Aberystwyth specimens the ribbings of the shell 

 are not very strong, and the colour olive-green to greyish. The chequered 

 appearance is due to the presence of a series of short white streaks dis- 



