146 THE MICROSCOPIST. 



41. Sphenosira. Fr. united into a straight filament; v. 

 wedge-shaped, at one end rounded, suddenly contracted 

 and produced ; aquatic. 



COHORT 11. NAVICULE^;. Fr. free, straight; v. equilat- 

 eral, or sometimes sigmoid. 



42. Navicula. Fr. single, free, straight ; v. oblong, lan- 

 ceolate, or elliptical, with a median line, a central and two 

 terminal nodules, and transversely or slightly radiant lines 

 resolvable into dots ; aquatic, marine, and fossil. 



43. Gyrosigma (Pleurosigma). Fr. as Navicula, but v. 

 .sigmoid ; aquatic and marine. 



44. Pinnularia. Fr. as Navicula, but transverse lines 

 continuous ; aquatic and marine. 



45. tStauroneis. Fr. a8 Navicula, but the median line 

 replaced by a stauros; aquatic and marine. 



46. Diadesmis. Fr. as Navicula, united into a straight 

 filament; aquatic. 



47. Amphiprora. Fr. free, solitary, or in pairs, con- 

 stricted in the middle; v. with a median keel, and a 

 median and terminal nodules, often twisted ; marine. 



48. Amphora. Fr. plano-convex, elliptical, oval or ob- 

 long, solitary, free or adnate, with a marginal line, and a 

 nodule or stauros on the flat Bide ; aquatic and marine. 



TRIBE II. VITTATJE. Fr. with vittse. 



f Valves without a Median Nodule. 



COHORT 12. LICMOPHORE^:. Fr. cuneate; vittae arched. 



49. Licmophora. Fr. cuneate, rounded at the broad 

 end, radiating from a branched stalk ; vittse curved (by 

 inflection of upper margins of valves) ; marine. 



50. Podosphenia. Fr. as Licmophora, but single or in 

 pairs, sessile on a thick but little branched pedicle; ma- 

 rine. 



