286 Postelstia 
The plant forms were less varied than the 
animal, being nearly all corallines. However, 
of these there were found in this single pool, 
six varieties, exclusive of the brown, pink, and 
ereen incrusting corallines. 
These six varieties were found attached to 
the rocks, or to limpet shells” Onythescorale 
lines, and on the limpets was found a small 
feathery green alga, but in very small quantities. 
The occurrence of so laree a ulm bermon 
forms in so small a pool, is accounted for by 
the fact that the illumination is unusually strong, 
that the pool is a low tide one, and that being 
shallow, the water becomes comparatively warm. 
The age of tide pools can be studied only 
relatively... Those. which are “smallinesniar 
disconnected and contain boulders possess 
the smallest number and the least variety of 
forms, and are probably the youngest. Regions 
of young pools are shown in Plate XXI. fig. 1. 
As the pools advance in age, they begin to 
attain irregularity of outline, and to lose their 
circular form, becoming more and more ellip- 
tical. Small pools lose their connecting ridges, 
