122 THE AMERICAN MONTHLY [June, 



Berkeleya^ pi. xxxi ; Homeocladia and Cymbosira^ xxix ; Toxonidia^ 

 Ixix ; A7tip}iiproi'alata^ ii, 20 ; Eucampia^ Ixvi ; Plagiogramma^ xlv ; 

 Podosira hormoides^ lix, 16; Pyxidicula lens^ Ixii, 10; Bacillaria^ 

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

 phora^ Rhipidophora^ and Clifnacosphcenia^ xxix ; Podosphenia^ Ixix ; 

 Striatella-And Rhabdonema^ li ; Gra7nmatophora serpe7itina^ xliv, 20 ; 

 Anaulus., Ixiv * Biddulphia piilchclla^yiZNx^ i ; Isthfnia^ c\y.\ Chceto- 

 ceros didy7n7is, Ixv, 14; Rhizoselenia stylifor77iis^ Ixviii, 29; Sy7'ing- 

 idiuT/i A77ie7'ica7ta^ ixiv, 38 ; Triceratiu77i favus^ xcix, 2 ; A7nphitetras 

 a7itediluvia7ta^ Ixiv, 39; Sy7ide7idriu7n^ Ixiii, 11. 



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

 of species it is a different 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 marine 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 genera have more than a dozen species 

 figured in Wolle's work. There 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 100 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, Amphipro7-a^ 18; 

 Biddulphia^ 35; RaphoTteis^ 13, and T7'ice7'atiur7i^ 74; total 140: 

 and the fossil has only 5, namely, Actittoptychus^ 27 ; Aulacodiscus^ 

 17; Aste7'077iphalos^ 13; Auliscus^ 26; CosciTiodzscus^ 52; while the 

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

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

 Ca77zpylodiscus, 42; CoccoTteis^ y:i\ Cymbella^ 25; Epithe7nia^ 15; 

 Eu7zotia^ 33; Fragilai'ia^ 20; Go77ipho7ie7na^ 23; Melosira^ 20; 

 SyTtedra, T^o\ Su7-irella^ 50; Stauroneis^ 23; Pleu7'osig7tza^ 22; 

 Nitzschia^ 53; while 7Vaw«:?^/a has the astonishing number oi 24J ; 

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

 vise him not to attack the genus Navicula Ji7'st., 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 motion of the dia- 

 toms struck me as impoi'tant, and as I do not remember to have seen 

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

 it. " The movements of the I)iato77iacece^^'' he says (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-i moved over i-200ths of an inch in a 

 second ; Pi7i7iula7-ia 7'adiosa., one of the slowest, over i-3400ths of an 

 inch in the same time ; and that the same period was occupied by Pi7t- 

 Ttularia oblonga in traversing i-2000ths of an inch, Nitzschia Ihiearis 



