392 Mr. J. Ralfs on the British Desmidiese. 



Tetmemorus differs from this genus in its elongated fronds and 

 emarginate extremities. 



Meneghini divides the plants belonging to this genus into two 

 sections, according as the surface is granulated or smooth. I have 

 used this division, but must remark that the mature plant should 

 be selected when we have to decide upon the absence of granules, 

 because they are frequently undeveloped in the young frondf. 



* Fronds rough with yearly granules. 



1. Cosmarium cylindricum. Fronds cylindrical, granulated; seg- 

 ments in the front view subquadrate, broadest at the end and nar- 

 rower at their junction ; end view circular. 



Mixed with other algse on the wet sides of a cave at Lamorna Cove 

 near Penzance. 



Fronds minute, cylindrical, about twice as long as broad, 

 rough with pearly granules which give a dentate appearance to 

 the outline; segments subquadrate, broadest at the end, and 

 slightly tapering towards their junction ; end view circular. 



This species may be known by the circular end view, by the 

 subquadrate appearance of the segments in the front view, and 

 by their junction along their entire breadth, so that no notch is 

 formed at the sides. 



Plate XI. fig. 1. C. cylindricum : a, front view ; b, empty frond ; c, end 

 view. 



2. C. orbiculatum. Segments spherical, rough with pearly granules. 

 Boggy pools near Dolgelley. 



Fronds very minute, composed of two spherical segments 

 which are rough with pearly granules, giving a dentate appear- 

 ance to the outline ; these are obscure or wanting on the neck- 

 like contraction which unites the segments. 



Plate XI. fig. 2. C. orbiculatum : a, front view ; b, empty frond ; c, trans- 

 verse view ; d, end view. 



3. C. ornatum. Fronds rough with pearly granules, deeply con- 

 stricted, the constriction forming a linear notch on each side ; end 

 view somewhat cruciform. 



In boggy pools about Dolgelley and Barmouth, J. R. ; Ashdown 

 Forest, Sussex, Mr. Jenner. 



Fronds about as large as those of C. margaritiferum, deeply 

 constricted in the middle ; the constriction forms a notch on each 

 side, which is nearly closed by the segments being in close appo- 

 sition for their entire breadth ; segments compressed, broader than 

 long, with an inflation down the middle, and which often slightly 

 projects at the end, where it is truncate. In the end view this in- 

 flation forms a rounded projection on each side, and thus appears 



f Meneghini has hinted at the probability of this in the following words : 

 " Superficies in nonnullis punctata vel granulato- (uti ainnt) margaritacea, 

 in caeteris lsevis, utrum vero hujusmodi diversitas ab cetate pendeat necne, 

 dijudicare nequeo." 



