1'22 THE AMERICAN MONTHLY [June, 



jBerkeleya^ pi. xxxi ; Hofneocladia and Cymbosira^ xxix ; Toxonidia^ 

 Ixix ; Amphiproralata^ ii, 20 ; Eucampia^ Ixvi ; Plagiogramma^ xlv ; 

 Podosira hor7noides^ lix, 16; Pyxidicula leits^ Ixii, 10; Bacillaria^ 

 Ixviii ; Podocystis^ li ; Raphoneis amphiceros^ xxxvii, 18 ; Licmo- 

 pho7'a^ Rhipidophora^ and Climacosphccnia^ xxix ; Podosphenia^ Ixlx ; 

 Striatella and Rhabdo?ie?)ia^ li ; Grammatophora serpentina^ xliv, 20 ; 

 Anaulus^ Ixiv " B idduIpJiia pulchella ^y.cs[\^ i ; Isthmia^cv^^ Chceto- 

 ceros didymus^ Ixv, 14 ; Rhizoselenia styliformis^ Ixviii, 29 ; Syi'ing- 

 idium Americana^ Ixiv, 38 ; Tricerathifn favtis^ xcix, 2 ; A??zphitetras 

 antediluviana^ Ixiv, 39; Syndendritim^ Ixiii, 11. 



So much for the genera. But when you come to the determination 

 of species it is a dift'erent matter. You must be prepared for work of 

 the hardest sort. The student will do well in that case to begin with 

 either the fossil or mai-ine genera and leave the fresh-water till the last, 

 for it is in the fresh-water division that he will find the question of 

 species most unmanageable. As a matter of curiosity I have taken the 

 trouble to see how many of the genei'a have more than a dozen species 

 figured in Wolle's work. Thei"e are 24 out of the 125. Omitting one 

 that has not been mentioned in this paper, the case stands thus : 24 

 genera re2:)resent nearly 1,000 of the 2,300 figured, the other lOO only 

 300 more, or 1,300. Those 24 genera then will tax the student because 

 each one contains a large number of species which must be distin- 

 guished. But the hard genera are not evenly distributed between the 

 three divisions. The marine has only 4, namely, Attiphiprora^ iS ; 

 Biddulphia^ 35; Raphoneis^ 13, and Pricerathcm^ 74! total 140: 

 and the fossil has only 5, namely, Actinoptychus ^ 27 ; Aulacodisctis, 

 17 ; Aster omphalos^ 13 ; Auliscus^ 26 ; Coscinodiscus^ 52 > while the 

 fresh water has no less than 15, and one of them is almost as large as 

 all of the marine and fossil genera put together. Amphora has 56 ; 

 Campylodisctis- 42; Cocconeis^ y:>\ Cymbella^ 25; Epitheinia^ 15 5 

 Eunotia^ 33; Fragilaria., 20; Go)npho7ieina^ 23; Melosira, 20; 

 Synedra,Tfi\ Surirella^ 50; Statironeis^ 23; Pleurosigma^ 22; 

 JVi'tzschia, 53; while A'az'/c?//a has the astonishing number oi 243 ; 

 total, 68j. If the beginner does not wish to go crazy therefore we ad- 

 vise him not to attack the genus Navicula Jirst^ or the fresh-water di- 

 vision at all until he has made the acquaintance of the marine or fossil 

 species, which are comparatively easy. 



But it is time to close this paper, which has already considerably 

 exceeded its original limits. Just a word, then, before I end. 



A remark in Rev. Wm. Smith's synopsis on the inotion of the dia- 

 toms struck iTie as important, and as I do not remember to have seen 

 the point stated so explicitly anywhere else, it may not be amiss to quote 

 it. " The movements of the Diatofnacece^'^ he aays (p. xxiii, vol. i), 

 "appear rapid and vivacious under the microscope ; but it must be re- 

 membered that the high powers usually employed in the observation of 

 these minute organisms magnify their motions as well as their bulk. 

 I have noted the movements of several species with the aid of an eye- 

 piece micrometer and a seconds watch, and found that one of the most 

 rapid, viz., Bacillaria paradoxa^ moved over i-200thsof an inch in a 

 second ; Pinmdaria radiosa^ one of the slowest, over i-3400ths of an 

 inch in the same time ; and that the same period was occupied by Pin- 

 mdaria oblonga in traversing i-20ooths of an inch, Nitzschia linearis 



