SYNEDRA 



307 



Tribe VII. Synedre^;. 



' Valves much elongated, with denticulate margins and short marginal costne Thalassiothrix. 



Valve with 



margins not 



denticulate. 



' Valves slender, with apices inflated into a head in an 

 unequal manner ; united together by the most 

 inflated apex, in a stellate form 



Valves 

 acicular or 

 bacillar, or 

 with apices 



equally 

 inflated. 



Asterionella. 



f Valves acicular or bacillar, with fine 

 regular puncta ; or valves very 

 attenuate between the centre and 

 the apices ; puncta rather coarse 

 and placed irregularly, especially' 

 in the median inflation . 



Synedra. 



t 



S= 

 II 



3gj 



GENUS 40. SYNEDRA EHR, 1831. 



Valves much elongated, more or less lanceolate 

 or linear, sometimes somewhat bent or undulate, fur- 

 nished with a hyaline median line or with a blank 

 space, sometimes obscure ; frequently with a central 

 pseudo-nodule and often with terminal nodules very 

 small and obscure ; striae transversal, never with 

 transverse costae. Frustule sessile on other plants. 



Endochrome formed of two laminae, denticulate at 

 the edges, or divided into straps and resting on their 

 valves at their median portion. 



Fig- 7i- 



Synedra ulna var. 



spathulifei a. 



