Because of great range of tide, the distribution of animals in the tidal zone 

 in relation to the elevation above raean low water presents a very interesting subject for 

 observation. A sequence of changes in the conposition of anir.ial coainunities, beginning 

 v;ith those occupying the upoermost positions and ending v/ith the forms living belov/ low- 

 water mark and never exposed at low tide repeats itself i/ith great regularity. Variations, 

 if any, are of minor nature and occur only v;hen typical rock habitat is modified by special 

 local conditions. Because of such regularity in vertical distribution, it is possible to 

 discuss it without repeating in detail the records of observations made at each station. 



The rocks of the uppermost tidal level are v/ell polished by the sea. They are 

 occu-oied by various species of Littorina and Her it a scabricostata Lam. Hundreds of 

 specinens of this gregarious gastropod are freq.uently found attached to the vertical 

 surfaces of rocks (Jig. ll). Slightly below this level, one encounters large cones of 

 Siphonaria gigas Sov:erby (Pig. 12) , and huge bariiacles, letraclita squamosa panamensis 

 Pilsbry. 



Horizontal surfaces of the rocks are pockmarked utth numerous pools varying in 

 diameter from a fev/ inches to several feet. The walls of these pools are very smooth. 

 Owing to crystal clearness of the sea water, the pools present an excellent opportuiiity 

 for a collector, for their bottoms are covered v;ith a nroltitude of gastropods, crabs, and 

 small fishes which find shelter there during the receding tide. In several instances a 

 brilliantly green carpet of Ealicystis sp. , grov/ing in great profusion, covered the bottoms 

 of such pools. They probably ov/e their origin to erosion by the sea v/ater, assisted by 

 mechanical grinding by pebbles and broken shells kept in motion by the surf (Fig. I3) . 



The vertical crevices of the rocks provided places of refuge to thousands of very 

 agile crabs, which were very difficult to catch; compound ascidiae, "medium-sized black sea 

 urchins, Diadena sp. , v;ith very sharp spines, and marine snails among vrhich Thais and 

 Purpura predominated. Tlmis crassa Blairsville was the most common gastropod of this 

 liabitat. 17ext in abundance was Leucozonia cingolata Lam. In places Thais crassa was 



f-^ 



■£* 



Figure 11 ITerita scabricostata on rocks near high-water mark. 



I'eari islands. 



23 



