DIATOMACE^E. 



[ 253 ] 



DIATOMACE^B. 



ment of the Diatomaceae, it appears impos- 

 sible at present to divide them satisfactorily. 

 The structure of the frustules of many 

 genera 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 

 strige for striae resolvable into dots, ^ con- 

 tinuous striae being costae 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 gelatinous 

 mass, nor in gelatinous tubes. 



Tribe I. Striatce. Frustules usually trans- 

 versely striate, but neither vittate nor 

 areolate. 



t Valves without a median nodule. 



Cohort 1. EuNOTmas. Fr. arcuate, single 

 or united into a straight filament. 



Epithemia. Fr. single or binate, with 

 transverse or slightly radiant striae, 

 some at least continuous ; no terminal 

 nodules; freshwater and marine (PL 16. 

 fig. 32). 



Eunotia. Fr. single or binate ; v. with 

 slightly radiant granular striae, and 

 terminal nodules j freshwater (PI. 51 . 

 fig. 27). 



Himantidium. Fr. as in Eunotia, but 

 united into a filament ; striae parallel, 

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



\Amphicampa. Fossil (PI. 18. figs. 11, 



12).] 



Coh. 2. MERIDIEJE. Fr. cuneate, single 

 or united into a curved or spiral band ; 

 valves with continuous or granular 

 striae. 



Meridion. Fr. cuneate, united into a 

 spiral band ; striae continuous ; fresh- 

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



Eucampia. Fr. united into an arched 

 band; v. punctate; marine (PI. 50. 

 fig. 10). _ 



[Oncosphenia. Fr. single, cuneate, un- 

 cinate at the narrow end ; striae granu- 

 lar ; freshwater.] 



Coh. 3. FRAGILAIIIE^. 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 ; striae continuous ; freshwater 

 and marine (PL 16. fig. 26J. 

 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). 

 Fragilaria. Fr. linear, united into a 

 straight close filament ; striae granular, 

 faint ; freshwater and marine (PI. 16. 

 fig. 83). 



Denticula. Fr. linear, single or binate, 

 rarely more united ; striae continuous ; 

 freshwater (PI. 16. fig. 25). 

 Odontidium. As Denticula, but fr. form- 

 ing a close filament; freshwater and 

 marine (PI. 17. fig. 14). 

 Cymatosira. Frustules united, margin 

 undulate in front view ; valves lanceo- 

 late, punctate, obtuse, without a lon- 

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

 Plagiogramma. Frustules quadrangular, 

 free, forming filaments ; valves with 

 two transverse-median and terminal 

 costae, the intervals transversely stri- 

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

 Coh. 4. MELOSIREJE. Fr. cylindrical, disk- 

 shaped, or globose; v. punctate, or 

 often with radiate continuous or gra- 

 nular striae. 



Cyclotella. Fr. disk-shaped, mostly soli- 

 tary ; 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 striae; 

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

 Podosira. Fr. united in small numbers, 

 cylindrical or spherical, fixed by a ter- 

 minal stalk ; v. hemispherical, punc- 

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

 [Mastogonia. Fr. single ; v. unequal, 

 angular, mammiform, circular at base, 

 without umbilical processes; angles 

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

 24,25). 



Pododiscus. Fr. single or united, with 

 a marginal stalk; v. circular, convex 

 (PL 17. fig. 16). ^ 



Pyxidicula. Fr. single or binate, free 



or sessile ; v. convex ; hoop absent ? ; 



freshwater and marine (PL 25. fig. 13). 



Stephanodiscus. Fr. single, disk-shaped ; 



