Recent researches in the dktomace^. 385 



Naviculacece. 



It is to be premised that, according to Dr. Pfitzer's con- 

 ceptions, this family is very much less comprehensive than 

 preceding writers have regarded it. Several genera hitherto 

 included among the Naviculacece are by our author separated 

 from it and transferred to other families. Navicula spharo- 

 pliora (Kiitz.) is made the type of a new genus, Atiomoeoneis, 

 and transferred to the Cymbelleee. The reason assigned is 

 that, though the outline is symmetrical, the valves are un- 

 symmetrical, in consequence of a lacuna in the striation on 

 the one side. The same peculiarity is noticeable in N. sculpta 

 (Ehren.), and N. bohemica (Ehren.), which forms are there- 

 fore 2)laced under this new genus. The genera Donkinia 

 (Pritch.), Amphijirora (Ehren.), and Amphitropis (Rab.), by 

 some comprehended in the Naviculacecs , are considered by 

 our author to be a group nearly related to the NitzschieiB, 

 while he regards Tooconidea (Donk.) as allied to the Cym- 

 bellece ; Berkley a and Rhaphidogloea to the Amphipleurece, 

 following in the latter case the suggestion of Grunow ; and 

 Mastogloia, according to the same authority, to the Cocco- 

 neidcB. However, he remarks in the case of the last-named 

 genus that, in consequence of the similarity that exists in the 

 structure of the auxospores, it seems more nearly related to 

 the Naviculacece. In some of these suggested transpositions 

 I feel inclined to concur ; there are others, however, which 

 appear to me open to serious objection. Still, further, a new 

 genus is suggested, named Neidium, of which I shall speak 

 presently. 



Navicula (Bory de St. Vine). — The distinctive character- 

 istic of this group is that in the process of cell-division the 

 endochrome plates, advancing along the wall of the cell, 

 move across from the girdle-bands upon the valves, and are 

 there separated by an oblique fissure. Whatever minor 

 differences are observable in the various species of Naviculce, 

 they all agree in this general feature that two mother-cells 

 co-operate in producing two auxospores. The latter term is 

 employed by Pfitzer to designate the result of conjugation, 

 and is suggested by the fact that the sporangial frustule is 

 ever about twice the size of the parent frustules. 



Neidium (gen. nov.) . — The features in which the species of 

 this genus diifer from the Naviculce proper are, first, that the 

 endochrome plates do not move, but are divided while still 

 remaining on the girdle-bands ; second, that the process of 

 division is efiected, not by an oblique fissure, but one parallel 

 to the longitudinal axis of the cell. 



VOL. XII.— -NEW SER. E E 



