38 Mr. HassalPs Notices of British Freshwater Conferva. 



ble in the more aged filaments wind round the interior of 

 each cell. 



I suspect that the mode of conjugation of this species is an- 

 gular, resembling that of Zygnema curvatum ; but I am not 

 certain that it is so. 



In a pond in Mr. Yorke's brick-field near Cheshunt. 

 Zygnema rivulare. Filaments seven or eight inches in length, 

 usually attached ; cells varying in length from eight to four 

 times their breadth, being at the period of conjugation 

 nearly four times as long as broad : three spiral tubes wind 

 round the interior of each cell, performing in those cells 

 whose length exceeds the diameter by eight times from six- 

 teen to twenty revolutions, and in those half as long as the 

 preceding half as many revolutions. 



The above is an exceedingly well-marked, and in this vici- 

 nity abundant species, and I am surprised that it has not be- 

 fore been described. It is most frequently attached either to 

 stones or wood in the New Barge and Lea rivers, but it is oc- 

 casionally found mixed up with other floating species in still 

 water. The filaments nearly equal in diameter those of Zy- 

 gnema deciminum, from which however it is readily distin- 

 guished by the greater length of its cells, and the presence of 

 three closely coiled spiral tubes: sometimes however there 

 are but two spirals, and then the resemblance to Zygnema de- 

 ciminum is much greater. 



Zygnema Grevilleanum. Filaments about equal in diameter to 

 those of Zygnema rivulare, round the interior of the cells, 

 which are generally about eight times as long as broad, 

 but frequently much longer ; two lax spiral tubes are dis- 

 posed so as to cross each other and describe in each cell 

 three or four oval spaces. 



To this species, probably the most elegant of the genus, 

 I have taken the liberty of assigning the name of Dr. Gre- 

 ville, author of the excellent 6 Algae Britannicae/ as a slight 

 mark of personal respect, as well as an acknowledgement of 

 the eminent services rendered by that gentleman to natural 

 history. 



Zygnema subventricosum. Diameter of the filaments about 

 equal to that of Z. commune ; cells at the period of conju- 

 gation about six times as long as broad, but prior to that 

 time frequently much longer : within the interior of each 

 cell a single spiral tube performs five or six revolutions, and 

 at the situation of the joints two short semicircular lines 

 are to be noticed : spores large, occasioning the cells in 

 which they are placed to assume a ventricose form. 



