100 THE AMERICAN MONTHLY [May, 



Blblariiim^ a ladder. In the foui-th, Periptera has circular valves and 

 marginal horns or spines; Dicladia, two horns witkhi the margin; 

 Hemiaulus ^ 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 mici'ographic. 

 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 pits. 



If there are no ra3'S it is Coscinodiscus^^ Odojtiodiscus^ 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- 

 noptycJnis^ Arachiioidiscus^ Asterolampra^ Asteromphalos^ Uali- 

 oiiyx^ Ojjiphalopelta^ or Heliopelta. 



You cannot mistake ylr(7'c/^;^cl/^/5c^/5; it is a spider's web. Otnpha- 

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

 single or double lines are not continuous. 



Is the surface wavy .'* 



It is Actinoptychus ox Heliopelta^ and teeth all around the margin 

 will make it the second. 



And so you have left these three : Asterolampra^ Asteroniphalos^ 

 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 Halio7tyx 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 1 3, 'namely: Cyniatosira, Doryphora^ 

 {z=Raphoneis) ^ Pleurodes?niuin (obscure), Hyalosira (probably 

 =Tetracyclus, Micrographic) , DiatoJiiella ^=Grainmatophora^ Mi- 

 crogvaphic, though H. L. S. retains it, Lithodesnihtni (doubtful), 

 Insilella (= a rubbed or worn Biddidphia')^ Coriiina^ Dickieia^ and 

 Syncyclia^ (not given by Smith), RhapJiidogloea {=Amphipleu?'a) ^ 

 and jSIic}-oinega {=^Sc/iizofie7/ia) . 



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



Frond — Berkeley a , Homeocladia. 



Nodtile — Cyinbosira^ Toxo7iidia^ Amphiprora. 



A^o Nodule — Euca77ipia^ Plagiogra77i777a^ Podosira^ Pododiscus^ 

 Pyxidicida^ Bacillaria^ Podocysiis^ Tryblio7iella^ Paphotieis. 



Heavy Pi7ies or sea77is — Plcl77iophora^ Podosphenia^ Phipido- 

 phora^ Cli77iacosphe7iia^ Striatella^ Rhabdo7ie77ia^ Anaulus^ Gra77Z- 

 77iatopho7'a. 



Processes or iiijlated A7igles — Biddulphia Isth77iia^ A77iphitetras^ 

 Chcetoceros^ Rhizoselenia^ Sv/-Z7tgidiu772, Triceratiu77i^ Sy7idctt- 

 driu77i. 



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



In the second Cy77ibosira has a bowed frustule, and the frustules are 

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

 A77iphiprora has winged valves and is consti-icted in front view. 



In the third, the frustules oi Euca77ipia form an arched band, those of 

 Bacillaria a series of rods. Plagiogra77i77ia has blank spaces at the 

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

 by stalks from the ce7itre of the valves. Pododiscrcs has a 77iargi7ial 



