42 THE AMERICAN MONTHLY [Feb., 



water and these were very active, showing their charac- 

 teristic motions and their usual color. On counting the 

 diatoms in the field of view in various parts of the slide, 

 I estimated that this drop contained over 250,000 indi- 

 vidual diatoms ; and as this was less than 1-5 of the frag- 

 ment of film under examination, I thought it safe to 

 conclude that each square inch of this film contained over 

 one hundred million diatoms. As the area covered by 

 this sheet of living diatoms was about 20 rods in width 

 and some 80 rods long, the number of individuals com- 

 posing it may well be reckoned inconceivable. I had 

 previously found agregations of similar diatoms cover- 

 ing a space of several feet in diameter, and had been 

 duly impressed by their prodigious numbers, but any- 

 thing like this I had never before seen. An acid treat- 

 ment soon demonstrated that the two smaller kinds did 

 not possess any silicious covering. They were as easily 

 dissolved and destroyed as desmids would have been by 

 the same treatment : the larger kind, however, survived 

 the operation and proved to be very small specimens of 

 Stauroneis salina. 



The soft ooze upon which rested this immense sheet of 

 living diatoms, and from which they had separated them- 

 selves and climbed up to enjoy the sunlight as soon as 

 the water had receded, was itself rich in varieties of 

 nearly all sizes ; but as these have been named in previ- 

 ous articles of this series in describing the diatoms of 

 Shell Beach, it is not necessary to specify them here. 

 The mud was also exceedingly rich in varieties of micros- 

 copic animals. 



Two weeks, later I visited Shell Beach again, and 

 found a similar brown sheet of diatoms spread over the 

 mud ; but examination showed important changes. The 

 two smallest forms had nearly disappeared, being found 

 only in small patches here and there ; the bulk of the 

 film being composed of two different sizes of Stauroneis 



