1891.] MICROSCOPICAL JOURNAL. 99 



these are the several characteristics : Plettrosignia is curved like an S ; 

 Statironeis has a cross-bar on both valves, Achnanthes only on one. 

 In Pinnularia the cross-lines resemble the teeth of a comb ; Amfhora 

 is inflated, and the frustule usually shows two knobs in the middle of 

 the flat side ; Gomphonema is wedge-shaped ; Cocconeis is elliptical, 

 and very broad for its length ; Cocconema is bent; Navicula has acute 

 ends, no cross-bar, and the lines are resolvable into dots ; Cocconema = 

 Cynibella with a stalk, and Achnaiithidutm == Achnanthes without a 

 stalk. 



But suppose your diatom has no nodule. If so, it belongs to the 

 other division, which includes the following i8 genera : Epithemia^ 

 Eunotia^ Himantidhtm^ Aleridion^ Diatoma^ Asterionella^ Fragi- 

 laria, Denticida^ Odontidium^ Cyclotella^ Melosira^ Stephanodiscus^ 

 Catnpylodisctis^ Stirirella^ Cyniatopleura^ Nitzschia^ Amphlpleiira, 

 Synedra. Of these Meridian can be readily told by the spiral form of 

 its united frustules. Now, as to the rest : 



Is your diatom shaped like a boxv? It is Epithemia, Ezinotia^ of 

 Hitnantidimn. If it is ribbed it is the first ; if it has knobs at the ends, 

 the second ; if the frustules form a filament, the third. 



Is it 7-ectangular in front viezv, and are several of the frustules con- 

 nected.? It IS e'lthev Eragilaria, Odontidinm^ Diatoma^ ov Asterio- 

 nella. Eragilaria forms a straight filament : Diatonia a zigzag chain ; 

 Asterionella a star ; in Odontidiuni the filament has a slightly uneven 

 outline ; in Denticula usually only two frustules are united. 



Is your diatom disk-shaped? It is either Cyclotella^ JSIelosira^ or 

 Stcphanodiscus. And if it forms a filament of jointed cylinders, it is 

 the second, while a fringe of little teeth will make it the third. 



Are the margiits odd? It is either Campylodiscns^ Surirella^ or 

 Cymatopleura. If the frustule is twisted, it is the first; if the valve 

 has a longitudinal line, it is the second ; dark bands on the valve dis- 

 tinguish the third. 



Finally, is it very long in proportion to its zvidth? It is either 

 Nitzschia^ Synedra^ or Amphipleiira. If the longitudinal line is not 

 in the middle, it is the first ; if the sides are nearly parallel, the second ; 

 if the longitudinal line has terminal expansions, the sides of which are 

 parallel, it is the third. 



Or the beginner may start equally well with the fossil forms, if he 

 has suitable slides for study. In that case he will have but 35 genera, 

 and even 5 of these may be passed over as being classed as doubtful or 

 obscure by the Micrographic, namely : Amphicampa, Amphipentas^ 

 Alastogonia^ Synibolophora^ and Stephanogonia. Divide the rest 

 thus: Breast-shaped — Hercotheca^ Goitiothecitom : Heavy lines or 

 sea?ns — Biblaritutt^ Stylobibliuni : Disk-shaped — Actinocyclus^ Ac- 

 tinoptychus^ Heliopelta^ Coscinodiscus, Arachnoidiscus, Astero- 

 latnpra, Asteromphalos^ Halioityx^ Odontodiscus^ Omphalopelta, 

 Systephania : with appendages — Eupodiscus^ Aidiscus ^ Hemiaulus^ 

 Periptera, Dicladia. 



Certainly, thus divided, this is no longer a formidable list, and you 

 notice that you are dealing with forms quite distinct from those of fresh 

 water. Those were mainly long forms ; these are chiefly round. 



In the first division marginal hairs distinguish Hercotheca. 



In the second, Stylobiblium is round, and the lines form zones, in 



