538 PLANKTON OF THE SYDNEY WATER-SUPPLY, 



been accorded the rank of a species as Cycl . Schroteri; the 

 form, however, is common to all species of Cyclotella. 



Genus Rhizosolenia (Ehr.) Brightwell. 



Rhiz. eriensis var. morsa W. &G. S. West. 

 (PI. Ivi., f. 29-31). 

 Long. corp. 29-140; lat. 7-U; crass. 2|-4; long. sp. 18-32/x. 

 Cf. W. &G. S. West, Trans. R. Soc. Edin. xli., pt. iii., 

 1905. Our frustules are somewhat smaller than European 

 specimens, and the spines very much shorter. Cf. Peragallo, 

 Monog. Rhiz., PI. i., f.l9, and W. & G. S. West, Plankt. of 

 some Irish Lakes, PL xi., f.5-7. All the freshwater Khizo- 

 iolenia form one species. 



Var. ZACHARiASi(Brun) mihi. 



Imperfect forms from the plankton. (PI. Ivi., f. 33-35). 



Long. Corp. 42-64; lat. I 4-16; long. sp. 18-28; lat. ami. 2 yu,. 



Perfect forms from Brisbane. (PI. ivi., f. 36-38). 



Long. Corp. 22-48; lat. U-lo; long. sp. 10-18; lat ann. 2 /x. 



8yn., Athtya Zuchanasi Brun. Cf. Apstein, Siisswasser- 

 plankton, p. 143, f.36. That the Brisbane forms are smaller 

 and with shorter spines is natural, owing to the higher tem- 

 perature of the water there. Pig. 33 should have a narrow 

 blank space down the centre ; in this, the longitudinal septum 

 forms later. A very interesting series of forms is here pre- 

 sented, illustrating the ved\i]Aic&i\on ol Ithizosolenia eriensis. 

 Fig. 29 is the simple form; it has apparently two modes ol 

 reproducing itself. In the first, tlie frustulc simply increases 

 in breadth towards the side remote from the spines, the 

 narrow blank space down the centre of such forms being 

 probably the place where the new membrane is forming. The 

 emarginate corners meanwhile grow outwards (fig. 33), to fill 

 in the rectangle ; and two new spines develop at the apices. 

 The other way of reduplication is exemplified in fig. 32, which 



