34 Bulletin Wisconsin Natural History Society [Vol. 11, Nos. 1-2 
V. COMPARISON OF THE FAUNAS OF THE DIFFERENT BEACHES 
At Nahant certain species are found everywhere between the 
tide marks provided they can find a suitable substratum. Balanus, 
Littorina litorea, Mytilus edulus, Idothea baltica and certain amphi- 
pods are examples of such hardy ubiquitous species — 'yet the first 
three are absent from the shifting sand beaches because there 
are no solid objects. The rock beaches have the largest number 
of species because they give ample shelter, solid substratum for 
attachment, pure cool water, and abundant food. Those animals 
that are unable to maintain themselves in the comparatively 
impure water on the mud flats drop out. Though a few char- 
acteristic species (Mya arenaria, Limulus polyphemus, Lumbri - 
conereis (?) come in to take their places, the great bulk of the 
fauna consists of the ubiquitous hardy forms also present on the 
rock beaches. 
Sandy shores have the most characteristic fauna of any of the 
beaches. Idothea baltica and certain amphipods are the only 
abundant animals present that are found in other situations. A 
large part of the population is made up of animals that are par- 
ticularly suited to life in the sand: ( Polyineces heros, Echinarach- 
nius parma , Venus mercenaria, (Jhiridotea caeca). 
[To be concluded ] 
Zoological Laboratory, 
University of Wisconsin, 
June 12, 1913. 
