6 MINUTE FORMS OF LIFE 



thus producing a periodicity of three seasons in the year. 

 During the winter and the early spring the Stephanodiscus 

 niagaras is the prevalent form, giving place to Melosira 

 crotonensis as the warm weather approaches, which soon yields 

 prominence to Tabellaria fenestrata, which continues the pre- 

 dominant form until fall, when the Melosira again appears, and 

 soon becomes equa'ly numerous, both forms, as cold weather ap- 

 proaches, becoming, scarcer, while the Steph-niagara? reappears 

 and rapidly increases in numbers, until at times it forms almost 

 the whole of some winter gatherings. The Actinocyclus niaga- 

 rae, so far as can yet be judged, appears most frequently in the 

 late winter and early spring gatherings; seldom or never in sum- 

 mer. The Amphiprora ornata and Rhizosolenia eriensis are 

 found occasionally at all seasons, but in greatest abundance in 

 the winter and early spring gatherings. During some weeks in 

 July and August, 1879, neither of these diatoms werefound, the 

 gatherings consisting almost exclusively of Tabellaria fenestrata. 

 The forms of Surirella and Cymatopleura are also most abundant 

 in the early spring gatherings. 



The influence of storms on so shallow a lake might well be 

 supposed to have considerable influence upon the contents of 

 the water supply, but so far as I have been able to observe, no 

 difference in the forms of diatoms found in the filterings is 

 caused by storms, but considerable mud is brought with them. 



This periodical abundance of the forms before mentioned, 

 the Actinocyclus, Rhizosolenia and Amphiprora, in the colder sea- 

 son, and their partial disappearance in the hottest season of the 

 year, and the uniform greater abundance of the Stephanodiscus 

 during the winter, also becoming scarce in summer, is strong 

 evidence, in my mind, that all these forms are survivors of the 

 cold seas, and were, perhaps, the chief forms then prevalent in 

 those waters; for which reasons the colder temperature of the 

 present lakes is most congenial. to them and best suited to their 

 development. 



As the changes that left our present lakes progressed, these 

 forms kept pace in their modification with that of their habitat, 

 new forms appearing and being introduced by various causes, 

 until the present condition has been reached, and I entertain no 

 doubt that if future changes should replace our lakes with arc- 



