SOME LITTORAL TKOCHID.^ AND LITTORINID^E IN CARDIGAN BAY. 117 



(c) Zone of small green Algce and many L. rudis. 



(d) ,, ,, few scattered Fucoids and a few L. rudis. 



(e) ,, ,, thick coating of Fucus and a few L. obtusata. 



(/) Base of wall, with pools at foot, with Enteromor'pha, Ulva, etc., and 

 L. littorea. 



M. crassa is local in Cardigan Bay, but often extremely abundant ; 

 and in the area south of Aberystwyth exhibits a striking peculiarity in 

 that it there breaks zone by following fresh-water influence, sometimes 

 as far as the low-water mark of spring-tide ; it is then often of remark- 

 able size. Proceeding southwards from Aberystwyth, the mouths of the 

 Rheidol and Ystwyth and a storm beach are passed, and a high 

 slaty cliff with eroded rocky foreshore is reached. About halfway 

 around this headland (Allt Wen), i.e. two miles from Aberystwyth, 

 G. umhilicatus puts in an appearance, at first only sporadically, two or 

 three small individuals here and there. As it becomes more numerous, 

 M. crassa also appears, and both increase rapidly to the south of the 

 headland, where the cliffs are composed of glacial drift. A number of 

 springs flow down, and in one or two localities fresh water percolates 

 through a shingle bank on the foreshore, and affects more or less the 

 whole of the lower rocky and stony portions of the tidal region, which is 

 there mainly composed of boulders covered to some extent by Algae. 

 Where there is fresh-water influence, M. crassa follows it. Streamlets 

 often follow a kind of channel or gutter, where the boulders lie less 

 thickly, and the shore level is somewhat lower. In these channels, 

 M. crassa swarms upon the nearly bare boulders, and extends almost 

 to low water. G. umhilicata is seldom to be observed within the fresh- 

 water influence, although abundant close by. Where a section of the 

 shore is affected by fresh water, M. crassa tends to become abundant 

 throughout. It appears to feed upon minute Algce, and I am indebted to 

 Dr. Fleure for a recent attempt to determine the food of this species : 

 all that could be made out, however, was that " the contents of the gut 

 consisted of finely triturated vegetable matter, too fragmentary for 

 identification." H. J. Fleure and M. M. Gettings state (Q.J.M.S., 1907), 

 " T. crassus [ilf . crassa^ is found to some extent with the previous species 

 \G. umbilicata], but it lives, for the most part, near high-tide level, so 

 much so that specimens may remain for a considerable time in corners 

 washed only by high spring-tides. It crawls over the rocks chiefly at half- 

 tide level, but is more lethargic than T. ohliquatus [G. umbilicata], and 

 less inclined to browse on the larger AlgcB. During stormy periods, 

 especially in winter, numbers may be found huddled in sheltered nooks. 



