634 PLANKTON OF THE SYDNEY WATER-SUPPLY, 



PI. Ivi., f.l7, indicates the formation of the tip of the trichome 

 from a broken filament. The cytoplasm contracts, and the 

 exposed cell- wall sloughs away. Undoubtedly the calyptra, 

 occasionally present, is of no specific value, being merely the 

 result of the incrassation of the apical cell. Broken filaments 

 show distinctly that, in Oscillaforia, the trichome consists of a 

 definite membranous tube with true septa; the cells are not 

 merely united together by mucus, as in Ariabcena. 



O. AMPHIBIA var. BIGRANULATA, n.var. 



Septa cellularum utrinque granulo amylaceo instructa. 

 Lat. trich. 2; alt. cell. 6-8 /x. 



BACILLARIEiE. 



Genus Vanheukckia Breb. 

 V. RH0MB0iDEs(Ehr.) Breb., var. SAX0NiCA(Rabh.). 

 Long. 50-90; lat. 13-16)u. 



Var. NEGLECTA(Thw.) mihi. 



Long. 50-70; lat. 8 fi. 



^yn., Schizonema neglfcta Thw., i\^av. gracilis va,r.7ieglecta{Thw.) 

 W. & G. S. West. 



I think it certain that this diatom is simply a narrow form of 

 V. rhomboides. 'i hey both abound in the waters round Sydney, 

 and are nearly always found together. There are several forms 

 of var. neglecta, of which two answer to V. rhomboides and its 

 var. saxonica respectively. Considerable variation also exists in 

 the character of the central line, and the central and apical 

 nodules are sometimes entirely wanting. This central line is not 

 a true raphe or slit in the membrane, Imt an incrassate columella 

 on the inner side of the valve, and enclosing the central nodule. 

 It is this lateral broadening that causes the central nodule in 

 most Vanheurckia forms, to appear excavated at each side. 



Var. HlTCHCOCKii(Ehr.) mihi, forma. 

 Foima utrinque in medio quam levissime inflata, apicibus oh- 

 tusioribus, paullo magis productis. 



