116 THE MICROSCOPE. Aug 



»• 



Note on Calcareous Algx from Michigan. 



D. p. PENH ALLOW. 



MCGILL UNIVERSITY, MONTREAL. 



In the latter part of the summer of 1891, Sir Wm. Daw- 

 son received from Mr. B. W. Thomas, of Chicago, some 

 very curious and interesting calcareous pebbles formed 

 by alga\ They were transferred to me with the request 

 that I should report upon the nature of the organisms 

 giving rise to them. These pebbles were found in con- 

 siderable numbers in certain Michigan ponds by Dr. J. 

 W. Velei, secretary of the Chicago Academy of Natural 

 Sciences. The specimens were found on a smooth, sandy 

 bottom, under about four feet of wfiter, and the collector 

 states that when fresh " they had so soft and slippery a 

 feeling" that he thought they were alive. Those which 

 reached me were said to be representative of the average 

 size. Tiiey were found to be irregularly ellipsoidal with 

 a thickness of about 40mm. and a diameter of about 

 60x80mm. Light ashy gray, the surface was found 

 chiefly smooth, but here and there with depressions of 

 various sizes, frequently merging into actual cavities. 

 The pebbles are not solid, but appear as a thickish shell- 

 like mass surrounding an irregular cavity, such as might 

 be conceived to arise from a progressive internal decay, 

 concurrently with an extension of the mass from witliout. 



Upon treating a portion of a pebble with acid to remove 

 the incrustation of lime, it became evident that we had to 

 deal not with an individual organism, but with a com- 

 munity possessing considerable diversity of plant 

 forms, while it was also evident that the growth of the 

 mass had involved the inclusion of a variety of foreign 

 bodies, some of which at least, miglit have served as a 

 base for the algal growth. 



From my notes taken at the time, it ai)pears that ow- 

 ing to the imperfect condition of some of the plants, it 



