» j
33. J nlves arcuate ........................................................................ H imantidium.
„ s tra ig h t............................................................................................... 34
34. Valves with a central s in u s .......................................................... E ucampia.
„ witliout a s in u s ................................................................................. 35
35. Valves delicately s tria te d .......................................................... F ragilaria.
„ distinctly costate ............................................................................. 36
36. Frustules united into a lengthened filament ...................... Odontidium.
„ lew, scarcely forming a filament ............................ Denticula.
37. Frustules compressed......... .......................................................................... 38
„ inflated .................................................................... T etracyclus.
38. Filament tenacious................................................................... Rhabdonema.
,, not tenacious............................................................... Striatella.
39. Frastules with internal septa .................................................................... 40
„ mthout septa ............................................................................. 41
40. Septa waved ................................................................. Grammatophora.
„ s tr a ig h t.............................................................................. T abellaría.
41. Frustules square or rhomboidal........................................... Amphitetras.
„ compressed ........................... 42
42. Frustules prismatic ....................................................................... Diatoma.
„ .................... subcylindi’ical .................................................................. 43
43. Angles of frustules rounded, slightly produced......................... I sthmia.
,, much produced ............................................................ Biddu lph ia .
44. Filament of few fru stu le s ............................................................... P odosira.
„ numerous frustules .................................................................. 45
45. Frustules with rounded extremities............................................. Melosira.
truncated ................................................................... Orthosira.
46. Frond indefinite ....................................................................... Mastogloia.
,, definite ................................................................................................. 47
47. Frond compressed............................................................................ Dick ie ia .
„ globular ......................................................................... Berkeleyia.
„ filamentous............................................................................................ 48
48. Frustules in row s .......................................................................................... 49
„ fasciculated .......................................................... H omceocladia.
49. Frustules those of a Cymbella................................................ E ncyonema.
„ „ N a v icu la .................................................................... 50
50. Filaments s im p le .................................................................... Colletonema.
„ bran ch ed .................................................................... Schizonema.
THE BRITISH DIATOMACEÆ.
‘ G e n u s 1. EPITHEMIA, Kiitz.
Frustules adherent, quadrilateral ; valves arcuate, furnished with transverse
canaliculi.
A section of the frustule in the present genus would present an
outline of varied form, either elliptical or quadrilateral, or that of two
nearly perfect circles united by an oblong isthmus ; the shape of the
frustule in the latter case may be compared, as it has been done by
Meneghini, to that of a coffee berry, or more correctly to a grain of
wheat, but in no case would the section present a trapezoidal outline,
the valves being always parallel or equidistant. This genus, as the
name implies, is characterized by the adherence of its frustules to
Ale-æ of a larger growth. This character is true of most of the species,
and even where not strictly applicable, as in E. gibba and others, we
detect a disposition to rely on a foreign body for support, their
frustules being usually imbedded in the mucus of some member of
the family of the Palmellaceæ.
The transverse costæ, so conspicuous in some species, I regard
as minute canals, which convey the nutrimental fluid to the surface
of the internal membrane. They are not modifications of the cellular
structure of the siliceous epiderm, otherwise, the striæ would not
pass, as they do, uninterruptedly over the entire surface of the valve.
Conjugation, and the formation of Sporangia in this genus, have
been noticed by Mr. Thwaites in E. túrgida and E. gibba, by myself
in these species and in jG. Sor'ex.