DIATOMACE.E. 



[ 253 ] 



DIATOMACE^. 



ment of the Diatomacese, it appears impos- 

 sible at present to divide tliem satisfactorily. 

 The structure of the frustules of many 

 o-enera is imperfectly known and described ; 

 and the supposed species have hitherto been 

 viewed rather in relation simply to their 

 differences in form, than to their specific 

 relations. 



The following synopsis, however, will 

 serve to aid in comparing the principal 

 genera scattered through the work. In it, 

 ■we have used fr. to denote the frustules as 

 seen in front view ; v. the valves ; granular 

 striffi for strife resolvable into dots, con- 

 tinuous striie being costfe or canaliculi. 

 The genera included within brackets are 

 not British, or have been imperfectly de- 

 scribed. 



Analysis of the Tribes and Genera. 



* Frustules not enveloped in a yelatinous 

 mass, nor in gelatinous tubes. 



Tribe 1. Striates. Frustules usually trans- 

 versely striate, but neither vittate nor 

 areolate. 



t Valves tvithout a mediati nodule. 



Cohort 1. ErNOTiE^. Fr. arcuate, single 

 or united into a straight filament. 



Epithemia. Fr. single or binate, with 

 transverse or slightly radiant strife, 

 some at least continuous ; no terminal 

 nodules ; freshwater and marine (PI. 16, 

 tig. 32). 



Eunotia. Fr. single or binate ; \. with 

 slightly radiant granular striae, and 

 terminal nodules ; freshwater (PL 51 , 

 fig. 27). 



Himantidium. Fr. as in Eunotia, but 

 imited into a filament ; strife parallel, 

 transverse ; freshwater (PI. 16. fig. 36). 



lAmphicain^ni. Fossil (PI. 18. figs. 11, 



12).] 

 Cob. 2. Meeidie^, Fr. cuneate, single 

 or united into a curved or spiral band ; 

 valves with continuous or granular 

 strife. 



Meridian. Fr. cuneate, united into a 

 spiral band ; strise continuous ; fresh- 

 water (PI. 16. fig. 28 ;_ PI. 17. fig. 7). 



Eucampiu. Fr. united into an arched 

 bfind; v. pimctate; marine (PI. 50, 



[^Oncosphema. Fr. smgle, cuneate, un- 

 cinate at the naiTow end ; strise granu- 

 lar ; freshwater.] 



Coh. 3. Feagilarie^. Fr. quadrilateral, 

 single or united into a filament or 

 chain ; v. with continuous or granular 

 striae. 

 Diatoma. Fr. linear or rectangular, united 

 by the angles so as to form a zigzag 

 chain ; strife continuous ; freshwater 

 and marine (PI. 16. fig. 26). 

 Asterionella. Fr. adherent by the ad- 

 jacent angles into a star-like filament ; 

 V. inflated at one or both ends ; fresh- 

 water (PI, 18. fig. 14). 

 Frayilaria. Fr. linear, laiited into a 

 straight close filament ; striae granular, 

 faint ; freshwater and marine (PL 16. 

 fig._3.3). 

 Denttcida. Fr. linear, single or binate, 

 rarelv more united ; striae continuous ; 

 fi-eshVater (PL 16. fig. 25). 

 Odontidium. As Denticula, but fr. form- 

 ing a close filament ; freshwater and 

 marine (PL 17. fig. 14). 

 Cpnatosira. Frustules imited, margin 

 undulate in front view ; valves lanceo- 

 late, punctate, obtuse, without a lon- 

 gitudinal line; marine (PI. 51. fig. 35). 

 Plagiogramma. Frustules quadrangular, 

 free, forming filaments ; valves with 

 two transverse-median and terminal 

 costfe, the intervals transversely stri- 

 ated; marine (PL 51. fig. 41). 

 Coh. 4, Melosiee^. Fr. cylindrical, disk- 

 shaped, or globose; v. punctate, or 

 often with radiate continuous or gra- 

 nular strife. 

 Cyclotella. Fr. disk-shaped, mostly soli- 

 tarv ; v. with radiate marginal striae ; 

 freshwater (PL 16. figs. 21, 22). 

 Melosira. Fr. cylindrical or spherical, 

 united into a filament ; v. punctate, or 

 with marginal radiate granular striaa ; 

 freshwater and marine (PL 17. figs. 5, 6), 

 Podosira. Fr. united in small numbers, 

 cylindrical or spherical, fixed by a ter- 

 minal stalli ; V. hemispherical, punc- 

 tate ; marine (PL 19. fig. 34). 

 [Mastoyonia. Fr. single ; v. unequal, 

 angular, mfimmiform, circular at base, 

 without umbilical processes; angles 

 radiating ; fossil (PL 18. figs. 23 a, b 

 24, 25). 

 PododiscHs. Fr. -single or united, with 

 a marginal stalk; v. circular, convex 

 (PL 17. tig. 16). _ 

 Pyxidicida. Fr. single or binate, free 

 or sessUe ; v. convex ; hoop absent ? ; 

 fi'eshwater and marine (PL '26. fig. 13). 

 Stephanodiscus, Fr. single, disk-shaped ; 



