174 



BRITISH DESMIDIE^. 



gradually attenuated ; the colour too of its empty frond is paler, and its striae 

 are not quite so crowded. 



I have frequently gathered the sporangium near Dolgelley, and Mr. Jenner 

 has gathered it in Sussex. It is very remarkable, being the only known ex- 

 ample of a geminate or bilobed sporangium. The fronds approach and couple 

 in the usual manner ; but instead of the contents of both fronds uniting in the 

 ordinary manner into a single body, a bilobed body is produced not unlike a 

 species of Costnarmtn in form. A distinct line is perceptible between the lobes 

 or portions, and each resembles a globular or oval body flattened at the junc- 

 tion, and is formed by the contents of the adjacent segments. The sporan- 

 gium is more closely comiected with the conjugated fronds than is the case in 

 species belonging to the first section, but the fronds are not permanently united 

 to it as in Closterium rostratiim. Although I have here called it bilobed, I 

 regard the sporangium as binate, rather than bilobed, because it readily sepa- 

 rates at the junction, each portion retaining the segments of the frond belong- 

 ing to it. 



Length of frond -^ of an inch ; greatest breadth g^i-g ; length of the bilobed 

 sporangium -^^-^ ; greatest breadth -^\-^ ; distance between the striae ^ g ^ ^ g . 



Tab. XXX. fig. 1. a. frond with endochrome ; b. empty frond; c. sporan- 

 gium. 



*** Frond either rostrate or minute, colourless and acicular ; sporan- 

 gium cruciform. (Stauroceras, Kiitz.) 



t Frond striated, tapering at each end into a distinct beak. 



18. C. Ralfsii (Breb.) ; frond stout, striated, curved, rapidly attenuated 

 into linear beaks which are shorter than the ventricose body. 



Closterium rostratum, Ralfs, in Jenner, Fl. of Tunhridge Wells, p. 196 (18-15) 



(not of Ehrenberg according to Brebisson) . 

 Closterium Ralfsii, Brebisson, in lit. (1845). 

 Stauroceras Ralfsii, Brebisson, in lit. (1846). 



Carnarvon and Dolgelley, J. R. INIidhurst ; near Tunbridge Wells, &c., 

 Mr. Jenner. Aberdeenshire, Dr. D^'eA/e. Near Manchester ; and Ambleside, 

 3Ir. Sidebotham. 



Falaise, Brebisson. 



Frond yellowish-browm, stout, six to eight times longer than broad ; the 

 upper margin convex, the lower concave, but ventricose at the centre ; extre- 

 mities tapering into a narrow Unear beak, which is curved dowiiwards, shorter 

 than the body, and obtuse at the apex. Vesicles disposed rather irregularly 

 in a single longitudinal series. Fillets generally obscure. 



The empty frond, which is firm, is reddish, especially at the ends. Striae 

 numerous, close, and distinct ; transverse suture solitary. 



This and the two following plants differ considerably from the other striated 

 species (Closterium lineatum in some respects excepted). The body of the 

 frond is somewhat lanceolate, being eqiially convex on both margins, and 

 tapering at each end into a linear or setaceous beak, which is curved down- 



