32 



THE AMERICAN MONTHLY 



[February, 



have a distinguishing character unlike 

 any diatom known to me. The 

 whole surface of the disc appears as 

 if covered with an opaque crust, not 

 of spicules, as represented by Mr. 

 Slack, but of minute chrystalline par- 

 ticles, which show nearly white by re- 

 flected light with powers of 500 to 1,000 

 (see The Lens, Jan. 1873, P- 29.) 



With transmitted light the diatom 

 is more or less opaque, with numer- 

 ous translucent apertures in each 

 aperture may be seen from i to 6 

 bright, bead-like dots or spherules. 

 The exterior crust appears to be easily 

 rubbed off, or dissolved by chemical 

 action. When this is partially or uni- 

 formly done over the whole surface, 

 it becomes E. Rogersii. I have au- 

 thentic specimens from my friend 

 Prof. H. L. Smith, and identical forms 

 of this dredging. In the next stage of 

 transformation the crust is, all but 

 a few remnants, removed ; we then 

 have a disc of polygonal reticulution, 

 a rough, transparent frame-work on 

 which that crust was adherent. 

 Through this maybe seen, not always, 

 a very hyaline disc with minute spher- 

 ules. These are what were seen at 

 first through the apertures in the 

 opaque disc ; now there is a fourth 

 state in the histology of this plant. 

 First, the crust ; the second, E. Ro- 

 gersii state ; the third, the frame-work ; 

 fourth, this has entirely disappeared, 

 and there is left a transparent disc 

 with minute spherules, in lines, 

 sparcely radiating from the centre to 

 the margin, looking like some figures 

 of Coscinodiscus. From solitary 

 specimens of each of these conditions 

 of Eupodiscus Argus these changes 

 could never have been worked out. 

 But I have not only seen numerous 

 specimens in each state, but also single 

 discs in which, on some of all condi- 

 tions could be seen. 



9. This is a peculiar form, narrow, 

 elongated, resembling both Gregory's 

 and Cleve's. I have seen no side view, 

 and the s. r. given by those authors 

 are unlike. Cleve notices the resem- 

 blance of his species to Gregory's. 



ID. Navicula pertnagna, B. is very 

 abundant. It has been oftener de- 

 composed than many other species in 

 the collection, but less so than E. 

 argils. The specific characters are 

 unchanged by the action. I found 

 but two frustules of Coscinodiscus, 

 neither of which I recognized, and 

 as I did not have Schmidt's Atlas 

 at hand, I would not attempt to give 

 specific names. 



There is one feature of this Tampa 

 Bay collection that often produces 

 curious effects. Most of the valves 

 of the discoid diatoms are of watch- 

 glass shape. When kept in bottles of 

 fluid, they are apt to pack themselves 

 together like a pile of watch-glasses. 

 This is more the case with these than 

 I ever saw in any other collection. 

 I have met with a Triceratiuju sym- 

 metrically placed in a disc, a disc in a 

 Triceratium, and once a disc, then a 

 Triceratiu?n, and outside of that an- 

 other disc. Such cases will be a puzzle 

 to novices using insufficient power ; 

 but when the two forms vary in size 

 and are not placed symmetrically, a 

 little acquaintance with diatoms tells 

 the observer what he has under the 

 microscope. When one of them is finely 

 marked, and both of the same size, as 

 I found in the case of an Auliscus 

 and an Actinopticus, there is a com- 

 bination of specific lines and marks 

 that may well perplex an expert until 

 he recognizes the true state of things. 

 Mr. Kitton, in the October number 

 of the Journal Royal Microscopical So- 

 ciety, shows how a diatomist was de- 

 ceived in such a case. 



I have used in this study princi- 

 pally a Tolles \ immersion of 1869 

 with a B- eye -piece, assisted by 

 Tolle's homogeneous \ and -^(^ and 

 iV water immersion, with a ^-inch 

 eye-piece. 



I will take this opportunity to dissent 

 from the position taken by some re- 

 cent writers on the microscope, that 

 a very fine low-power objective with a 

 high-power eye-piece, is equivalent to, 

 or will do as good work as, a high op- 

 jective with a lower power eye-piece. 



