448 PEOF. A. DENDT ON PONTOBOLBOS, 



filaments, it would make it all the more difficult to account for 

 ■the origin of these layers, for Lynghya is supposed to reproduce 

 by means of motile filaments and not by means of gonidia or 

 cocci, and the zoogloea hypothesis of the origin of the granular 

 layers would accordingly be untenable. The elongated form of 

 the cells in the filaments, however, seems to exclude the idea 

 that we are dealing with a true Oscillarian in the case of 

 PontohoJhos. True Oscillarian filaments do occasionally occur 

 ;as foreign bodies (PL 27. fig. 8 a), but they offer a striking contrast 

 in appearance to the vastly more numerous filaments which I 

 .consider as bacterial (fig. 8). 



Another comparison has been suggested to me by Mr. E. M. 

 Laing, M.A., B.Sc, and that is with the calcareous pebbles 

 formed by Scliizothrix fasciculata as described and figured by 

 Murray *. These pebbles were found at the bottom of a pond 

 in Michigan, and I extract the following account of them from 

 Mr. Murray's interesting paper : — " 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 struc- 

 ture. 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 predominating kind was 

 clearly a species of Schizothrix, while mixed with it there were 

 other forms f? notably filaments of Stigonema and DicJiotJirix. 

 I examined portions here and there from a number of the 

 pebbles, and in all cases found the strong stout sheaths and 

 filaments of ScTiizotJirix composing by far the greater part of 

 the decalcified mass. At the surface the Schizothricc filaments 

 had been clearly alive when the pebbles were gathered, while 

 ]iearer the centre older sheaths only were found. It was 

 apparent that the Schizothrix died off internally while fresh 

 crops were produced on the surface adding to the growth of 

 the pebble. . . . S. fasciculata is known from the countries of 

 Central Europe, and forms small, stony, cushion-like calcareous 

 masses. The filaments are always entangled where they are 

 incrusted, but on the surface, where Lhey are more or less free, 

 there is very little entanglement and the filaments are almost 

 straight and parallel. The ordinary incrustation formed by this 



* Phycological Memoirs, April 1895. 



t Compare the occasional occurrence of Oscillarians in Fontobolbos {vide 

 PI. 27. fig. 8 a). 



