168 BRITISH DESMIDIEiE. 



broad; extremities tapering to a subacute apex. At the centre of the lower 

 margin is a slight protuberance, which however is often nearly obsolete. Fillets 

 frequently obscure ; vesicles few, large, and disposed in a single series. The 

 empty frond has generally a pale straw-colour tinge, and a transverse suture is 

 usually visible at the centre. 



Clostermm Leibleinii much resembles a young specimen of C. moniliferum ; 

 but it is smaller and more curved, the ends are more acute, and the empty 

 frond is usually coloured. 



The variety /3. is nearly intermediate between this and the following species. 



I have gathered the sporangia at Penzance and Dolgelley ; Mr. Jenner has 

 found them in Sussex, and Mr. Thwaites near Bristol. They are orbicular, 

 and but loosely connected with the empty segments of the fronds. 



Length of frond from -jiy to -^^ of an inch ; greatest breadth from xsVa 

 to 5^^ ; length of sporangium from y^Vo ^^ "ozr '■> breadth from 2^75" ^^ T2Ts- 



Tab. XXVIII. fig. 4. a, c,f, i, h. different states of frond with endochrome ; 

 b, d, e. empty fronds ; ff, h, I. sporangia. 



9. C. Diana (Ehr,) ; fronds smooth, slender-crescent-shaped ; extre- 

 mities tapering ; apex subacute ; lower margin not inflated ; vesicles 

 in a single series. 



Closterium ruficeps, Ehr. Abh. der Berl. Ak. 1831, p. 67. 

 Closteriiim Biance, Ehr. Lifus. p. 92. t. .5. f. 17 (1838). Kiitzing, Phy. 

 Germ. p. 130. Hassall, Brit. Freshwater Alg. p. 371. 



Sussex, Mr. Jenner. Dolgelley, J. R. Aberdeenshire, Dr. Dickie and 

 Mr. P. Grant. Cheshire and Ambleside, Mr. Sidebotham. 



Germany, Ehrenberg . Falaise, Brebisson. 



Frond slender, six to eight times longer than broad, curved ; extremities 

 tapering to a subacute point. The lower margin is concave and not inflated 

 at the centre. Fillets obscure ; vesicles disposed in a single longitudinal 

 series. 



The empty frond has a pale straw-colour tint, and a transverse suture at the 

 centre, where, as in C. Leibleinii, the segments easily separate. 



This plant differs from C Leibleinii in being longer, more slender and less 

 curved, and in having no projection at the centre of the lower margin ; yet I 

 have doubts whether they are really distinct, as I have seen some specimens 

 apparently intermediate. 



Length between ends of frond yij of an inch ; greatest breadth yaVs"- 



Tab. XXVIII. fig. 5. a, b. fronds with endochrome ; c. empty frond. 



ttt Frond nearly straight, scarcely attenuated ; ends truncate ; longi- 

 tudinal stria none or indistinct. 



10. C. didymotocum (Corda) ; frond nearly straight, broadly linear ; 

 extremities slightly tapering ; ends truncate, reddish ; fillets ob- 

 scure. 



