118 CHAS. L. WALTON. 



■often with a number of Littorina littorea as companions. As is well 

 known, the spire of T. ohliquatus is much lower than that of T. crassus, 

 and this is probably correlated with the greater activity of the former 

 in the shore zone, where a high spire would give too much purchase to 

 a side blow from a wave." Recent observations lead to the same con- 

 clusions : the animals crowd the tops of the boulders in calm summer 

 weather, and in winter tend to occupy the sides and angles of boulders 

 and reef-pools. 



It is evident that in this region Trochus is extending its range 

 northward, Qaite recently Mr. W. Whitehouse informed me that he 

 had seen specimens on the Aberystwyth side of Allt Wen. This I 

 found to be the case, as I found a colony of G. umhilicata and one 

 specimen of G. cineraria about the spot indicated. This is well to the 

 north of any other record, but is close to an isolated patch of boulder 

 clay. I have examined this spot at intervals ever since 1906, and 

 Prof. Fleure knew it well for some years before that, and no Trochi 

 have ever been seen there. 



A remarkable influence of fresh water in larger volume upon the littoral 

 Mollusca has been noted, about the mouths of several of the smaller 

 rivers. It is particularly noticeable for some half-mile or so on either 

 side of the mouth of the Afon Wyre, near Llanrhystyd. The cliffs there- 

 abouts are very low, and composed of glacial drift, with the usual result 

 that the foreshore consists of shingle, while the lower portion of the 

 tidal region is occupied by boulders of medium size. These extend to 

 low-water mark, but pass into sand and shingle both to the N. and to 

 the S. ; and this boulder area is fairly well covered with Algal 

 growth. The river debouches upon a shingle bed, and spreads out in a 

 fan-like manner over the boulders ; and, to judge by the distribution of 

 mussels, would appear, as usual, to trend up-coast, at least during some 

 states of the tide. To the south of the river where the boulder area 

 commences (and, indeed, throughout the area) the zones of L. neritoides 

 and the upper portion of that of L. rudis are absent owing to the presence 

 of shingle. The boulders are at first considerably incrusted with Sabel- 

 laria, and no Mollusca are present except a few specimens of Patella, 

 probably owing to the proximity of much sand and shingle. As the 

 boulder area widens G. umhilicata appears, and rapidly becomes abundant. 

 That lack of shelter and attrition are the excluding factors is borne out 

 by the fact that, where Gihbula first appears, the shells of many are much 

 worn and eroded, and in several cases actually broken. I examined 

 certainly 1000 Gibbulas, and only discovered two specimens of cineraria. 



