3 o8 



SYNEDRA. 



I. EUSYNEDRA. Valves regularly striated, scarcely or not 

 inflated at the middle and without sulci. 



Strias 

 extending 

 throughout 

 nearly all 

 the 

 breadth of 

 the valve ; 

 pseudo- 

 nodule 

 linear. 



Pseudo-nodule annular, central 



( Valve 



furnished 



with 

 a pseudo- j Pseudo-nodule eccentric 

 nodule. I. 



S. pulchella. 

 S. Vaucheria. 



Valves 

 without a 

 pseudo- 

 nodule. 



Valve 



furnished 



with a 



median 



hyaline 



space. 



Valves 



without 



hyaline 



space. 



Valves 

 narrowly 

 linear- 

 lanceolate. 



Strias well marked, about 13 

 in 1 c.d.m.; apices very 

 slightly rostrate-capitate 



Stria? fine, from 16 to 18 in 1 

 c.d.m. ; apices slightly ros- 

 trate-capitate 



Valve narrowly linear ; apices more or less 

 rostrate, or rostrate-capitate 



Valves 



with 



apices not 



capitate ; 



marine 



species. 



Striae 

 robust, 

 about 

 9 in 1 

 c.d.m. 



Valve large, 

 lanceolate 



linear- 



Valve very small, 

 very narrowly linear 

 lanceolate 



Apices 



capitate ; 



fresh 



water 



species. 



Strias fine, about 18 in 1 c.d.m. ; 

 valve rather broadly lanceo- 

 late ... 



Valve very large and very 

 robust, with ap'ees strongly 

 capitate, head triangular 



S. Acus. 



S. radians. 

 S. Ulna. 

 S. Gallionii. 



S. investien8. 



S. barbatula. 



S. capitata. 



Apices 

 not 

 with 

 trian- 

 gular 

 head. 



Valve long and robust S. Ulna var. div. 



( Raphe nar- 

 row ; strias 

 very deli- 

 cate . S. famelica. 



Valve 

 small. 



Raphe ra- 

 ther broad, 

 striae very 

 robust . S. amphlcephala. 



Strias ( Strias marginal ; no coarse beads on the margins of the valve 

 marginal 



or pseudo- 

 raphe 



S. affinis. 



Strias of valve furnished with coarse beads, between which are fine strias 



lanceolate. |^ scarcely visible 



. S. Nitzschioides. 



II. ARDISSONIA. Valves furnished with two or more sulci. 



Valves 



having 



only two 



sulci. 



Pseudo- 

 raphe 



distinct ; 

 valve 



narrow. 



Pseudo- 



raphe 



obscure ; 



strias 

 delicate. 



I 



Sulci very close to the margins S. fulgens. 

 Sulci distant from the margins S. crystallina. 



Raphe very distinct, strias robust 



I 



^ No pseudo-raphe, valve very broad . 



Valve with more than two sulci, some submarginal, others placed in the 

 i. middle of the valve .... 



S. superba. 

 S. baculus. 



S. robusta 



