102 ^Ir. C. Mereschkowsky on Sellaphora. 



On the Affinities of the Genus Sellaphora. 



In order to show the right place of this genus in the system 

 of diatoms it is necessary to give here a short exposition of a 

 new classification of the Eaphidian diatoms, based on the inner 

 structure, and especially on the endochrome. This system 

 will be more fully explained in ray paper " Rapport pr^limi- 

 naire sur la Structure interieure des Diatomees." For the 

 present I simply give its main features. 



I divide the Khaphidoa? into three groups : — 



I. Archaideae. 

 II. Monoplacatae. 

 III. Polyplacatae. 



The Arehaidefe represent the central group from which 

 have sprung on one side the two remaining groups of 

 Raphidian diatoms (Monoplacatfe and Polyplacatje) and on 

 the other the Carinatse (Surirelloideje, Nitzschioideai) ; the 

 actually living representatives of the Archaideaj are the genera 

 Auricula, Amphiprora, Amphoropsis (s. emend.), Epithemta, 

 Stauronella *. 



The Polyplacatfe are characterized by a symmetrical 

 disposition of the plates, usually two in number (Diplacatteor 

 Naviculacese), sometimes four (Tetraplacatfe, including the 

 genera Scoliotropis, Achnanthidium, MaatogJoia, and certain 

 species of Amphora)^ or even more, as in the Okedeniete f. 

 The Pleurotropidcfe (comprising two sections or families, 

 Pleurosigmeai and Tropidoidea?) form a separate division of 

 the Pol} placate representing an independent branch of the 

 common stem \. 



Finally, we have the Monoplacatse, which are characterized 

 by a single plate disposed asymmetrically. This group is 

 mainly composed of such forms as Cymhella, Gomphonenviy 

 Bhoicosphenia, Anomceoneis, and Brebissonia, in which the 

 plate is situated on the dorsal connecting-zone with a pyre- 

 noid in its centre. On account of this latter characteristic I 

 call this group, which is a very natural one, Pyrenophorese. 

 There are, however, several other groups of Monoplacatas 

 besides the Pyrenophorefe, as, for instance, the Cocconeideoe 



* See my note " On Stauronella, a new Genus of Diatoms," Ann. & 

 Mag. ^'at. Ilist. 1901, vol. \iii. p. 424. 



+ See my note " On Okedeni'a," t. c. p. 415. 



t The inner structure of IMeuro.<«i^'m(a' and Tropidoidoro kavos no 

 doubt ns to their near relation.'ihip. The Tropidoideae have notliin-r in 

 common with Amjj/iijJfora or the Nitzschiefip, and are not related to them 

 in any respirt. 



