PLANKTON OF THE WOODS HOLE REGION 109 
As most rules have exceptions, so, too, an exception to this rule is often found 
at Woods Hole during the summer months when the oceanic diatom (Rhizosolemia 
semispina) occurs in such abundance that almost every other form of animal and 
plant life disappears for a time. The occurrence of this interesting species will be 
discussed later. 
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The diatom spores are no doubt at all times exceedingly numerous in local 
waters and are carried about by the currents and winds. When conditions become 
favorable for those already present or those transported to a harbor or river mouth 
