RECENT RESEARCHES IN THE DIATOMACEiE. 85 



As regards the construction of auxospores, Thwaites in- 

 forms us tliat in this genus, as well as that of Cymbella, two 

 mother-cells develope two auxospores lying parallel to them. 

 Thwaites made this observation in the case of a species 

 related to Gomphonema dichotomum, Sm., in G. dichotomum, 

 G.tenellum, G. olivaceum ; Pfitzer, in the last-named species. 

 An actual union of the two primordial mother-cells does not 

 occur, but only a diffusing of the contents through the gelati- 

 nous investment. When the auxospores have nearly reached 

 their definitive length they develope a fine membrane, 

 within which the valves are formed one after the other. 

 They are at first strongly arched and bent on the longitudinal 

 axis ; the striation developes itself clearly in a direction pro- 

 ceeding from the centre towards the ends. The firstling- 

 cells at first have girdle-bands as narrow as those in Naviciila, 

 the outer larger valve, even in its earlier stage, embracing 

 the smaller inner valve. After the second division, out of 

 the bent-valved firstling-cells spring normal cells with two 

 straight valves, just as in the case of Navicula. The plane 

 of separation in the firstling-cell is at a right angle to that of 

 the mother-cell from which it has sprung — the valves of the 

 former being seen when the girdle-band of the latter is turned 

 towards the observer. 



Rhoicosphenia. 



This may be regarded as a Gomphonema unsymmetrical 

 in its three dimensions. In addition to the Avant of symmetry 

 in the longitudinal and transverse axis, there occurs in this 

 genus a bending in the plane of separation, and also an un- 

 similarity of the two valves, of which one only — namely, the 

 concave — possesses a central nodule ; besides, the fillets 

 which occur on the upper end of Gomphonema are here most 

 strongly developed. In the primordial cell no remarkable 

 distinction is found compared with Gomphonema ; but it is 

 far otherwise with Achnanthes, Achnanthidium, and Cocconeis, 

 to which genera Grunow and Kabenhorst attached it, Rho- 

 icosphenia curvata (Kiitz.), Grun., and R. marina (Kiitz.), 

 possess a single endochrome-plate, the middle line of which 

 lies on the plane of one girdle-band, covering the two valves, 

 and even folding itself over upon the other girdle-band. 

 Viewed in this aspect it appears broken into four parts, the 

 division between Uwo of these parts being observable on the 

 F.V. Normally this occurs in a Rhoicosphenia lying on one 

 valve on the same side of the upper and under valve. The 

 middle portion of the valves is for the most part covered 

 with endochrome, which is not the case in Gomphonema, A 



