52 THE AMEEICAN MONTHLY [Feb 



Those not marked common are rare. Prof. Cragin's 

 list furnished by Wolle does not agree wi'th the above. 

 Wolle enumerates: 



Cymbellacistula — more frequently fresh-water than 



brackish. 

 Navicula peregrina — brackish water ; N. tenella and 



N. lanceolata which may be forms of Sohizonema 



and hence marine. 

 Coscinodiscus woodwardi,B. — most decidedly marine. 

 Melosiragranulata,(E.) E,. — brackish or marine water. 

 Of these, Cymbella cistula is always found in fresh- 

 water now. The Navicula, I have none of. Coscinodiscus 

 woodwardi, Eilenstein, is figured in Schmidt's Atlas der 

 Diatomaceen-kunde, 1878, plate 61, but no description has 

 been published. It is seen in H. L. Smith's Species 

 Diatomacearum Typicse Studiis. Schmidt's figure looks 

 like C. centralis, C. Gr. E. C. minor, C. G. E. is found 

 in fresh-water. It is then supposed to be Melosira distans 

 F. T. K. C. woodwardi is only found in the fossil con- 

 dition. It looks like and may be only an end view of M. 

 distans. In my specimen of the Loup Fork marl, I see 

 disk-like forms that may have been classed as Coscinodis- 

 cus. M. granulata may be M. distans. In fact, they are 

 indistinguishable the one from the other. Thus the ma- 

 rine habitat of the Loup Fork marl fades away and fresh- 

 water habitat is established in it. But then, these brack- 

 ish diatoms may be there still. In fact, it is extremely 

 likely that they may be found, for brackish water merely 

 results from fresh-water and has not been introduced 

 from the ocean at all. It must be remembered that, in a 

 gathering several feet away, the forms may differ very 

 much. Such is the case now. So that the reasoning of 

 the marine or brackish nature of the Loup Fork marl from 

 the forms one finds which is usual may be and is very un- 

 reliable. We must find larger plants and animals, brack- 

 ish or marine, to rank the layer, and they are fresh-water. 



