ZYGNEMA. 139 



their diameter and length being very considerable. Cells, 

 when in a state of conjugation, a little longer than broad, 

 prior to which, however, they are frequently not half so 

 long as broad ; winding round the interior of these are 

 about eight spiral threads, the granules in them being small. 

 Sporangia almost circular, flattened. 



Z. maximum Hassall, in Annals of Nat. Hist. vol. x. p. 36. 



Hab. Cheshunt, Hertfordsliire, and the country adjacent ; 

 Notting Hill : A. II H. Waldron and Wortliing ; Mr. 

 Jenner. Graham Castle : Major Martin. 



This Is one of the finest as well as largest of all the Zygne- 

 mata hitherto described, the diameter of the filaments greatly- 

 exceeding those of Z. nitidum, tlie Conjugata -princeps of 

 Vauclier. It is found only in ponds and dykes whose waters 

 are deep and permanent, and it does not conjugate until near 

 the end of the summer. The only other Zygnema with which 

 it could possibly be confounded is Z. scrratum, between which 

 and the present plant several well marked differences exist, 

 as will presently be shown. Two varieties of this plant were 

 described by me In the 11th vol. of " Annals and INIagazIne 

 of Natural History," p. 432. both of which I now feel as- 

 sured are quite distinct. The first of these is Z. alternatum. 



2. Zygnema alteenatum Hass. 

 Plate XX. 



Char. Filaments of the same diameter as those of the preceding 

 species. Cells rather longer than broad. Spires 7iumerous, 

 granules small, conjugation alternate, that is, every second 

 pair of cells alone conjoin. 

 Hassall, Annals of Nat. Hist. vol. xi. p. 432. Zygnema 

 Decaisne, In INIcraoire sur la Classification dcs Algues, 

 An. des Sclen. Nat. 

 Hab. Cheshunt, Hertfordshire. A. II. H. 

 This is one of the rarest species of the genus, and I am not 

 sure that I liave ever met with it more than once. Tlie mode 

 of conjugation of the cells, tlie alternate ones only uniting, ia 



