10 



ANALYSIS OF THE GENERA. 



33. Valves arcuate Himantidium. 



„ straight oa 



34. Valves with a central sinus Eucampia. 



„ without a smus 35 



35. Valves dehcately striated Fragilaria 



„ distmctly costate . 3g 



36. Frustules imited into a lengthened filament Odontidium. 



„ few, scarcely forming a filament Denticula. 



37. Frustules compressed 33 



„ inflated Tetracyclus. 



38. Filament tenacious Rhabdonema. 



not tenacious Striatella. 



39. Frustules with internal septa 4q 



„ without septa 4 j 



40. Septa waved Grammatophora. 



„ straight Tabellaria. 



41. Frustules square or rhomboidal Amphitetras, 



„ compressed 42 



42. Frustules prismatic Diatoma. 



„ subcyhndrical 43 



43. Angles of frustules rounded, slightly produced Isthmia. 



„ much produced Biddulphia. 



44. Filament of few frustules Podosira. 



„ numerous frustules 45 



45. Frustules with rounded extremities Melosira, 



" truncated Orthosira. 



46. Frond indefinite Mastogloia. 



„ definite 47 



47. Frond compressed Dickieia. 



„ globular Berkeleyia. 



„ filamentous 48 



48. Frustules in rows 49 



fasciculated Homceocladia. 



49. Frustules those of a Cymbella Encyonema. 



M ,, Navicula 5q 



50. FUaments simple Colletonema. 



branched Schizonema. 



