136 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Followed seaward M. balthica dies out and Tellina tenuis 

 takes its place both in the Dyfi Estuary and on the Traeth 

 Lavan, at about low water of ordinary spring tides, although 

 I found them to overlap at about this level, below which Tellina 

 also occurred in places apparently too unstable for Cardium ; 

 I can only agree with. Woodward that the shell form of Tellina 

 enables this species to survive where the heavier and thicker 

 shelled form cannot adapt themselves.* Petersent states 

 that, " Transition stages between the successive communities 

 are doubtless found as a rule in nature," but seem to occupy 

 a very small part of the whole. 



The following was the surface succession on a line taken 

 across the Traeth Lavan, near Aber, on May 27th, 1919, 

 roughly 1,000 yards and extending from the foreshore to the 

 " commercial " cockle beds. Broadly, four faunistic zones 

 were traversed : — 



1. Steep, stony slope, 50 yards (more or less). 



2. Muddy sand, level and wet, 150 yards. 



3. Sand, fine, firm and less muddy and damp, 700-800 yards. 



4. Coarse sand, a wide expanse. 



Between 3 and 4 there was, in places, a shallow channel 

 50-100 yards wide. 



1. Contains several sub-zones ; in its upper drier 

 portion were sand-hoppers ; in the lower, Fucus, Entero- 

 morpha ; Littorina littorea (very abundant), L. rudis, 

 P. stagnalis and Carcinus. Littorina may be considered 

 the dominant form. 



2. Ulva, Ruppia maritima ; L. littorea, L. rudis, Carcinus, 

 P. stagnalis, very common, and Corophium voluator. 

 Paludestrina the dominant form. 



3. Paludestrina (fairly common), Arenicola (common), 

 Macoma, Cardium, etc. 



4. Cardium (large), Tellina tenuis, Arenicola. 



* See B. B. Woodward, Proc. Malac. Soc, VII, 5, 1907 ; also Hunt, A. R. 

 Journ. Linn. Soc, XVIII, pp. 262-74. 

 t Op. cit., p. 13. 



