10 The American Naturalist. [January, 



called " Fucoids," and the problematical organisms in general 

 are mostly found in strata whose appearance indicates disposi- 

 tion in shallow water. Now this is in just the position where 

 algaB might be expected to occur, but it is also the place where 

 the chances of preservation are fewest. This seems to be con- 

 clusively shown by the almost complete absence of true ani- 

 mal remains from strata where the problematical organisms 

 are most abundant. While fossils occur both above and below 

 this horizon, and frequently in the greatest abundance, the 

 actual layers where " Fucoids" are found are notorious for 

 their barrenness. The fragments which are found attest the 

 abrading power of the water and we again see the small chance 

 cellular organisms would have of being preserved, when cal- 

 careous bodies of animals are ground to fragments. 



On the other hand it should be remembered that shallow 

 flats, exposed, it may be, to the air twice a <l ; ,v or even' 

 ered with a slight depth of water, are admimblv situated 'to 

 receive and retain impressions left by crawling animal 'forms 

 Rain drop impressions, too, could be preserved? as well as mud 

 cracks and the excavations made by rills of water on •, '.|„,, in- 

 shore. These have all oecu red. Rain drop iinpre-ion< -un 

 cracked earth, rill marks on the sla,re and the hum w^'i-m il' 

 of worms and molluscs, are all known from various geological 

 horizons. But true algae in the older rocks are rare indeed • 

 and the most of those described as such take their place among 

 the much discussed problematic organisms. The probabilities 

 hat rue alga, are included among the long list of species re- 

 ferred to as plants is almost infinitely small- while on the 

 contrary the chances that what have been so considered are 

 referable to tracks, trails or inorganic causea are almost in 

 finitely great. Nathorst has pointed out that an alga? in sink- 

 ing to the bottom of the water, if sufficiently solid to be ore- 

 served, would not make a depression in the mini but rather an 

 elevation. In reality the depression is what is found in the u 

 of the stratum, while the elevation or cast occurs on the bottom 

 of the next overlying stratum of rock. 



