Figure 4.—Schematic diagram of zonation on south-east wall of Red Beach Cave—summer. Vertical scale = twice horizontal.

considerably owing to a curve of the west wall so far in towards the centre of the aperture that there is scarcely room to squeeze sideways into the innermost recesses. The south wall is tiered in a series of terraces 0.2-0.5 metres wide from above E.H.W S. to about M. T.L for the first 10 metres, the wall then becoming steep and more or less vertical. This portion is continually moist, probably through seepage from the cliffs above, and the moisture is responsible for reflecting a certain amount of light across to the more seaward facing north wall. Differential indirect lighting effects between walls are less pronounced than in the Red Beach cave; however, the flat ledges between M. H.W N. and E. H.W. S. on the south wall, together with the more constant moisture, encourage growth of certain algae—e.g., Bostrychia arbuscula and Ptilothamnion pectinatum, which are absent from the vertical northern side.

The absence of Peyssonelia from the south wall could conceivably be related to the occurrence of direct summer sunshine on that wall, but this seems unlikely in view of the fact that the same species is known to occur in sunlit pools in the open midlittoral. Among the crustose red algae Peyssonelia and basal Corallina seem less shade-tolerant than Hildenbrandtia, which is the last algal survivor as far in as 24 metres. extending from the cave roof to just below high water mark, as far as could be judged. At this depth it was necessary to shine a torch or light a match to examine the walls in mid-winter. Beyond the narrow crack was found a scattered assemblage of tiny Micrelenchus dilatus, a univalve with an opalescent shell typically living among kelp at extreme low water. Such a population may be expected to range over almost any part of the cave according to its immediate preference or requirements. The same applies to Melaraphe, Lepsiella (and Onchidella), the ranges on the charts being those at the time of observation. Shells of Elminius plicatus show a marked