286 iPo s t e I s i a 



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 

 green incrusting corallines. 



These six varieties were found attached to 

 the rocks, or to limpet shells. On the coral- 

 lines, and on the limpets was found a small 

 feathery green alga, but in very small quantities. 



The occurrence of so large a number of 

 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 small, regular, 

 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. i. 



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, 



