A detailed examination of rock crevices on a selected shore in Anglesey has been made. The tidal levels and percentage exposures of numerous cracks having been established, the contents and certain conditions within the crevice were investigated. These included a mechanical analysis and a qualitative study of the soil particles, estimates of the moisture content, of the humidity of the air trapped in the crevice, and of the organic carbon content of the substrate. A few readings were taken of the air and rock temperatures at the different levels.
Qualitative samples of the fauna were taken from crevices a t different tidal levels and from different parts of a single crevice. During the survey forty-nine species were recorded from this habitat, thirty-eight of which were plentiful enough for their distribution to be plotted.
For the sake of convenience the fauna has been divided into a marine and a terrestrial component. With the exception of Littorina rudis, all animals belonging to the marine component extended down to E.L.W.S., whereas their upper limits were found mainly in one of the three levels of M.L.W.S. M.L.W.N. to E.L.W.N., or M.H.W.N. to E.H.W.N.
All the members of the terrestrial component showed a well defined upper and lower limit between tidal marks. Three groups were defined by the extent of their distribution.
An examination of the spatial distribution within the crevice also showed differences between the two component parts of the fauna. The marine organisms, with the exception of Eulalia viridis were limited to the outer regions of the crevice, whereas the terrestrial forms were recorded as more common in the inner zone of the crevices at the h e r levels of the beach but more evenly distributed throughout the crevices higher up the shore.
Some observations on the feeding habits of the fauna of the crevice have been made in the field and supplemented by further observations made on animals in the laboratory. A diagram showing the possible feeding relationships of the members of the fauna a t different levels is presented.
The distribution of this fauna is discussed in relation to the conditions within the crevice; a few remarks on the probable stages in the development of the community are added.