XIII. 



CALCAREOUS PEBBLES FORMED BY 



MR. E. GROVE has been good enough to give me several large calcareous 

 pebbles collected by Dr. J. W. Velie, Secretary of the Chicago Academy 

 of Sciences, who found them in a pond in Michigan. Mr. B. W. Thomas, 

 of Chicago, had sent them to Mr. Grove with a request for his opinion as to 

 their nature, and Mr. Grove forwarded them to me. I am indebted to Mr. 

 Thomas for a further supply. The pebbles were found resting free on a 

 sandy bottom under three to eight feet of clear water in a pond separated 

 from Lake Michigan by a sand bar. The specimens vary in size from 

 an inch to three inches and a half in diameter, are hollow in the interior 

 and show a stratified or concentric zoned structure. On decalcifying a 

 portion of one, I found that it was composed of a densely interwoven 

 mass of filaments evidently not all of the same nature, but the pre- 

 dominating kind was clearly a species of Sckizotknx, while mixed with 

 it there were other forms, notably filaments of Stigonema and Dichothrix. 

 I examined portions here and there from a number of the pebbles, and 

 in all cases found the strong stout sheaths and filaments of Schizothrix 

 composing by far the greater part of the decalcified mass. At the surface 

 the Schizothrix filaments had been clearly alive when the pebbles were 

 gathered, while nearer the centre older sheaths only were found. It was 

 apparent that the SchizotJirix died off internally while fresh crops were 

 produced on the surface adding to the growth of the pebble. On com- 

 paring this Schizothrix with the figures and description in M. Gomont's 

 admirable Monographic des Oscillarices, I found a difficulty in identifying 

 it, but none in placing it near certain other species. I accordingly con- 

 sulted him, and he had no hesitation in identifying Schizothrix fasciculata 



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