100 THE AMERICAN MONTHLY [May, 



Biblariuni^ u ladder. In the fourth, Periptera has circular valves and 

 marginal horns or spines ; Dicladia^ two horns within the margin ; 

 Hemiatilus ^ two marginal horns and the valves not circular ; Eupodis- 

 cus and Auliscus have short horns or knobs, and the latter is sculp- 

 tured. Aulacodiscus is classed with Eupodiscus by the niicrographic. 

 The characteristic is the furrow connecting the process with the centre 

 of the valve. 



In the third division the valves are circular and marked with small 

 dots or jiits. 



If there are no rays it is Coscinodiscus^^ Odontodiscus^ or Syste- 

 phania. Teeth within the margin will make the third ; on the margin, 

 the second. 



Or rays being present, it is one of these eight : Actinocyclus^ Acti- 

 Hopiychus^ Arachnoidiscus^ Asterolampra^ Aster omphalos^ Hali- 

 onyx^ Omphalopelta^ or Heliopelta. 



You cannot mistake yIrac/i^/ci/'<^/5c?/5; it is a spider's web. Ompha- 

 lopelta has teeth within the rays. In Actinocyclus some of the radiating 

 single or double lines are not continuous. 



Is the surface wavy ? 



It is Actitioptychus OY Heliopelta^ and teeth all around the margin 

 will make it the second. 



And so you have left these three : Asterolampra^ Astci-ornphalos ^ 

 Halionyx. If the rays suggest a star it is either the first or the second, 

 and if the star is perfect it is the first. In Halionyx the rays start from 

 a transparent star or a ring of shorter rays. 



Or again the student may take the marine forms. Here are 42 genera, 

 from which he may omit 13, namely: Cyniatosira^ Doryphora^ 

 {=Raphoneis) ^ Pleurodesmi/im (obscure), Hyalosira (probably 

 =Tetracyclus, Micrographic), Diatomella {= Granitnatophora^ Mi- 

 crographic, though H. L. S. retains it, Pit/iodesmium (doubtful), 

 Insilella (= a rubbed or worn Biddulphia) ^ Corinna, Dickieia^ and 

 Syncyclia^ (not given by Smith), Rhaphidogloea {=Amp/tipleura) ^ 

 and Micr omega { = Schizonetna). 



This leaves 30genera, which may be ranged under these five divisions : 



Frond — Berkeleya , Ho7neocladia . 



Nodule— Cynibosira^ Toxonidia., Aniphiprora. 



'No Nodule — Eucanipia^ Plagiogramma ^ Podosira^ Pododisc7/s, 

 Pyxidicula^ Bacillaria^ Podocystis, Tryblionella^ Raphotieis. 



Heavy Lines or seams — Bicimophora^ Podosphenia^ K/iipidu- 

 phora^ Cliniacosphcnia^ Striatclla^ Rhabdonema^ Anaulus^ Gram- 

 matophora. 



Processes or inflated Angles — Biddulphia Isthmia^ Amphitetras^ 

 Chcetoceros^ RJiizoselenia^ Syringidium^ Triceratium^ Synden- 

 drizim. 



In the first division Berkeleya has a frond rounded at the base. 



In the second Cymbosira has a bowed frustule, and the frustules are 

 connected endwise by stalks. Toxonidea has a bowed median line, 

 Aniphiprora has winged valves and is constricted in front view. 



In the third, the frustules oi Eucatnpia form an arched band, those of 

 Bacillaria a series of rods. Plagiogramma has blank spaces at the 

 middle and the ends of the valves. Podosira has the frustules connected 

 by stalks from the centre of the valves. Pododisc?is has a margifial 



