BRITISH DESMIDIE^. 169 



a. Empty frond divided into four portions by three transverse 

 lines or sutures. 



Clostermm didymotocxim, Corda, Almanack de Carlsbad, 1835, p. 125. t. 5. 



f. 64, 65. Brebissow, in lit. cum icone et specimine. 

 Closterium subrectum, Brebisson, Alg. Fal. p. 59. t. 8 (1835), Kiitzing, 



Phijcoloyia Germanica, p. 131. 



/3. Baillyanum (Breb.) ; frond smaller with a suture only at the 



middle. 



Closterium Baillyanum, Brebisson, in lit. cum icone et specimine (1845). 



a. Rare. Dolgelley, J. R. 



/3. Common. Dolgelley and Penzance, J. R. Sussex ; Surrey ; and 

 Hampshire, 3Ir. Jenner. Aberdeenshire, Mr. P. Grant. Ambleside, Mr. 

 Sidebotham. 



Germany, Corda, Kiitzing. a. et /3. Falaise, Brebisson. 



Frond six to ten times longer than broad, stout, nearly straight, equal ex- 

 cept at the extremities, which taper more or less ; ends truncate, fillets obscure ; 

 vesicles in a single longitudinal series ; terminal globules and moving granules 

 distinct. 



Empty frond reddish, especially at the ends, where the colour is conspi- 

 cuous even before the endochrome has collapsed. Faint striae may generally 

 be detected in old specimens. 



The variety fl. is by M. de Brebisson considered a distinct species. The 

 only chfference I can discover is in the number of sutures, and I doubt whether 

 any dependence can be placed on this character. Closterium Baillyanum is 

 also usually smaller than a, but not invariably, and before the escape of the 

 endochrome I am unable to distinguish them even as varieties. 



Closterium didymotocum may be known from all the preceding species by 

 its straight frond combined with trmicate ends. It differs from C. striolatum, 

 C. intermedium, and C. angustatum, in the absence or indistinctness of its 

 striae and the upward inclination of the lower margin at the extremities. 



Length of frond m /3. -^ of an inch ; breadth -g\^. 



Tab. XXVIII. fig. 7. a. frond of a. with endochrome; b. empty frond; 

 c. frond of j8. with endochrome ; d. empty frond. 



tttt Empty frond distinctly striated, mostly coloured. 



11. C. attenuatum (Ehr.) ; frond curvedj attenuated, suddenly con- 

 tracted at the end into a conical point ; empty frond reddish, 

 faintly striated. 



Closterium attenuatum, Ehrenberg, Tnfusor. p. 94. t. 6. f. 4 (1838). Kiitzing, 

 Phycologia Germanica, p. 131. Brebisson, in lit. cum icone. 

 Chiltington Common near Pulborough ; Midhurst ; Ashdown Forest ; near 

 Tmihridge AVells, &c., Sussex ; Reigate, Surrey, Mr. Jenner. Dolgelley and 

 Penzance, /. R. Ambleside, Mr. Sidebotham. 



Berlin, Ehrenberg . Falaise, Brebisson. 

 . Frond rather larger than that of Closterium striolatum, eight to twelve 



