SOME LITTORAL TROCHID.E AND LITTORINID.E IN CARDIGAN BAY. 121 



posed in a spiral sequence ; these white marks, alternating in each turn 

 of the spiral produce a chequered appearance. The usual forms, sitnilis 

 and Iwvis of Jeffreys, were both present in the same locality. The variety 

 patula, Thorpe, is sometimes abundant on steep rocks at the foot of cliffs, 

 somewhat influenced by fresh-water springs, and may be yellow, olive- 

 green, pale greenish white, black and white in bands, or pinkish ; com- 

 fressa, Jeffreys, occurs a little to the south of Aberystwyth. 



The complete sequence of species of Littorina may be interrupted, 

 and a barren area may occur between the cliff rocks and the lower tidal 

 reefs, owing to a belt of shingle. The result of this truncation of zone 

 is that the upper portion of the rudis zone may be absent except here and 

 there where conditions allow ; while below the break, rudis and littorea 

 may be found together in quantity, giving the impression that the latter 

 is above its usual horizon. 



Dr. Fleure considers that L. rudis spawns largely in the shelter afforded 

 by the interior of dead Balanus, and I have found many of the young of 

 both rudis and littoralis so small as to be just recognizable clustered 

 within the angles of empty Balanus. The absence of Balanus from the 

 rudis zone may also have a limiting efEect, locally, on the distribution of 

 the species. A curious and interesting form of Molluscan association is 

 common on some parts of the cockle beds of the Dyfi estuary, and usually 

 on the more muddy areas. Scattered over from one to several acres in 

 such places, are clusters, or groups, composed primarily of one or more 

 mussels, attached to either various dead shells which lie upon the surface, 

 or to one or more living cockles of varying sizes. The latter occupy their 

 usual positions in the sand, thus anchoring the mussels. Upon the cockles 

 grow trailing tufts of various green Algw, and clinging to the mass are 

 generally several individuals of L. littorea and L, rudis, more often the 

 former. Within the mass, amongst the byssal threads of the mussels, 

 one or more specimens of Macoma baltJiica are frequently to be found. 

 There are often a dozen individuals, belonging to five species of Mollusca, 

 involved in these clusters. 



Factors Governing Distribution. 

 The facts concerning distribution, association, etc., are matters of 

 observation, record, and comparison ; the causes of distribution are 

 problems of a much more difficult nature, and require careful considera- 

 tion following wide investigation. With regard to the distribution of 

 the Trochidee in Cardigan Bay, it is not yet possible to state that the 

 problem has been completely elucidated. It would appear, however, 



