$ 
. 304 RECENT RESEARCHES IN THE DIATOMACER. 
3. Cymbella. 4. Cocconema. 5. Encyonema.—These three genera 
correspond in this common feature, that the portions of the valves at the 
opposite sides of the longitudinal axis are unsymmetrical. One margin 
is ever more convex than the other, in many forms one margin is concave. 
The section of the cell is sometimes rectangular, sometimes strongly 
trapezoid ; a longitudinal line and nodules occur as in Navicula, Brebis- 
Cymbella Ehrenbergii, Kütz. ; sometimes moniliform, as in Cocconema 
asperum, Ehrenb., in which case there are depressions arranged in linear 
order 
described by Ehrenberg, and regarded by him as a spermatie gland, pre- 
sents itself on the more s rongly arehed girdle-band, and thence, inter- 
posed between the cell-wall and the endochrome-plate, passes over slightly 
on ve | v 
The middle plasm-mass observable on the valves in Cymbella gastroides, 
Kiitz., and Cocconema Cistula, Hempr., is broader on the convex than 
the concave side, and the reverse in Cymbella cuspidata, Kitz. In the 
smaller forms, Cymbella scotica, W. Sm. for insta ce, no distinction 1n 
breadth is noticeable. In the last-named species the endocrine 1s 
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species investigated by Thwaites, but also in Cocconema parvum, W. Sm., 
and Encyonema prostratum, Ralfs. Lastly Lüders gave a full description 
n Cocconema Cistula. The two cells co-operating 1n this 
ivision of a 
* m» . m 
plasm-masses Superimposed one on the other, which then conjugate 1m 
TS, 
