evident, and in Figure 10 is shown an arrangement of algae and animals upon a much creviced slate. Barnacles of two distinct types and mussels, mingled with a growth of Endocladia muricata, appear in this
view, but the general grouping is less clearly concentric. Nevertheless the Endocladia zone is pretty well defined as a mid-tide algal society and sometimes shows sharp demarcation when favorably situated for a pure growth.
Among the high-tide algae—those occupying the upper zones—Pelvetia is an interesting form. It occurs at the same levels adopted by