346 FRESHWATER AL^ OF LISMORE DISTRICT, 



in gatherings, the frustules are generally found separate, or in 

 groups of 4-8. 



Navicula minima Grun. (PI. xlv., f.l3.) 

 Long. 14-16, lat. valv. 5-6; crass, frust. 3 /x. 



Navicula atomus Nag. (PI. xlv., f. 14, 15.) 

 Long. 6-9, lat. valv. 3-4, plerumque 4; crass, frust. 3 fx. 



Navicula exilissima Grun. (PI. xlv., f.l6.) 

 Long. 10-14, lat. valv. 4-5; crass, frust. 3 /x. 



Navicula Flotowii Grun. (PI. xlv., f. 17.) 

 Long. 14-22, lat. valv. 5-6, ap. 2-3; crass, frust. 3 /x. 

 Lismore, all four forms (178, 186); the last three (176, 178); 

 JVav. atomus {17Q, 178, 186, 192, 196). 



Of. Van Heurck, Diatomacese, pp. 227-230, P1.5, f.229, 231, 

 234, 238. Nav. Flotowii is the only one of these four that is 

 recognised by either Cleve or Van Heurck as Diadesmis. Here, 

 however, I find them all in long ribbons (diam. 6, 8, 10, 11, 12, 13, 

 14 // in sample No.186). Also, they all exhibit a similar appear- 

 ance under the microscope, and are all found in quantity associ- 

 ated together. There seems, therefore, to be some connection 

 between them. 



Genus Vanheurckia Breb. 

 Vanheurckia rhomboides (Ehr.) Breb. (PI. xlv., f.l8.) 

 Long. 68, lat. 16, ap. circ. 4 //.. 

 Lismore (185). 



Cf. Donkin, Brit. Diat., PI. vi., f.ll. A very rare form of the 

 genus; this is only my second record of it. The other specimen, 

 from the Sydney Water Supply, measured 90 x 16/x. The apex 

 is simply rounded, not rostrate at all. 



Vanh. vulgaris (Thw.) Van Heurck. (PI. xlv., f.19-21.) 



Long. 40-50, lat. 10-14, ap. 3-4 /x. 



Lismore (185). 



Thwaites, Ann. Mag. Nat. Hist., Ser. 2, Vol. i., Pl.xii., f.Hl-5. 

 Not Van Heurck, Diatom., p. 239, f.39 (= V. viridula) nor P1.5, 

 f.252. Thwaites says "lanceolate, suddenly narrowed near the 



