76 JOURNAL OF THE [July, 



of the diatom being a plant instead of belonging to the infusorial 

 group, as Ehrenberg had at first placed it. 



"As tending to prove that the minute diatoms, visible through, 

 or upon, or in the body of the disc, are not casual surface debris 

 of the deposit in which the discs grew, I would mention the fol- 

 lowing as a very delicate test. In one of the discs on the slide 

 there is a minute Dictyocha partly overlapped by a minute 

 diatom of the Naviciila didyma shape. To view either of these 

 small diatoms distinctly, a material movement of the fine ad- 

 justment must be made ; for while one is in focus the other is out 

 of focus, thus showing that they are not on the same superficial 

 plane. Again, many of the minute diatoms offer a strong con- 

 trast with the transparent disc, as would be the case where dia- 

 toms are seen in such a refractive medium as liquid sulphur, in 

 which the diatoms look black by contrast with the enclosing 

 medium ; or where mediums are compounded with phosphorus, 

 giving the highest refractive value. If a lens of high power, say 

 a j'g or a y^g-, is used, these minute diatoms will prove of greater 

 interest than the larger discs with which they are associated. 

 My experience with the new diatom material has given me an en- 

 tirely novel field of interest and study, which is within the reach 

 of all whose forte is to unravel new truths and evolve new lines 

 of thought in relation to the histology of the diatom. 



" In having placed this new source of diatoms on record with 

 the New- York Microscopical Society, we have types of diato- 

 maceous deposits from the three principal geological eras of the 

 tertiary period : the eocene, by the diatoms from St. Stephens, 

 Alabama (tripoli); themiocene, from the Florida phosphate clay; 

 and the pliocene, from the clays encountered at a depth of seven 

 hundred feet in the Mobile artesian wells ; not to mention the 

 recent, or living, species surviving through these long periods of 

 sedimentary deposition. In summing up the result of a limited 

 amount of study of this miocene fossil diatomaceous clay, I find 

 the following genera represented : Craspedodiscus, Coscinodiscus, 

 Actinopti'cus, Inceratiu/n, Biddulp/iia, Afelosira, Navicula^ Ra- 

 p/ioueis, Pleurosigma, Synedra, etc." 



Dr. Arthur Mead Edwards, of Newark, New Jersey, being 

 introduced by the President, addressed the Society on "The 

 Occurrence of Marine Diatoms in Fresh Water." This address 



