Mr. C. Mereschkowsky on Sellaphora. 193 



or Ileteroidefe, whicli re|)resent a literal branch composed of 

 degenerated forms ( Cocconeis aiul Microneis, Cleve) . Another 

 group is formed by the new genus Cotenuhi *, characterized 

 by the jihite being phiced not on the dorsal but on the ventral 

 connecting-zone and by the absence of a pyrenoid. And, 

 finally, we have a fourth group of the Monoplacataj represented 

 by Sellaphora, where the ])late rests not on the couuecting- 

 zone, but on the surface of one of the valves. 



The main idea of this new system lays in the division of 

 the Kaphidian diatoms (other than the Archaide^) into two 

 entirely independent principal branches, one with a single 

 asymmetric plate and the otlier with two or more symmetric 

 plates. It seems to me quite impossible to admit that iu the 

 course of evolution of diatoms forms with two or more 

 symmelric plates could have originated from forms with one 

 asymmetrical plate or vice uer5<^, and this not once but several 

 times. The supposition that all Monoplacataj have had a 

 common ancestor with a single asymmetrical plate is much 

 more natural and in conformity with the constancy of the 

 number and position of chromatophores in diatoms. The 

 single plate may have changed its position in the frustule 

 and lost its pyrenoid, thus producing the groups Cocconeideai, 

 Cattnulaceaij and Sellaphorete. 



The genus Sellaphora is therefore to be regarded as a 

 separate group (family) of the MonoplacatEe. 



The new classification of the Raphidian diatoms is made 

 clear by the following table : — 



RAPHIDE^. 



I. Polyplacatae. 



A. Diplacatae. 



1. Naviculaceae (Navicula, Diploneis, Caloneis, Sec ), with two 



zonal plates. 



2. Cleviacese {Clevia-\), with two plates resting on both 



valves. 



B. Tetraplacatfe. 



1. ScoliotropidesB {Scoliotropis, Achna7ithUliiim, Achnanthes). 



2. Ma^togluieie. 



C. OlcedeniecB. 



1). Pleurotropideae. 



1. Pleurosiginefe. 



2. Tropidoideae. 



* See my note "Sur Cafenula, un nouveau Genre de Diatom^es," 

 Scripta Botanica, St. Petersburg, 1902, fasc. xL\. 



t Under this name I propose to unite the Naviculese Punctatne and 

 I.vratiB, which have all two plates placed on the surface of both valves. 

 The genus Fseudoumphijjrora, which has a similar structure, belongs also 

 to the familv Cleviacere. 



