112 BRITISH DESMIDIE^. 



siders that the central protuberances will afford better distinctive 

 marks of some species. However, I have seen too few examples to 

 decide this question. 



One plant {X. octocorne, Ehr.) without central projections, and in 

 other respects different from the true species of Xanthidium, I have re- 

 tained here, because I am unable to assign it a more proper situation. 



* Spines divided at the apex. 



1. X. armatum (Breb.); segments broadest at the base; spines short, 

 stout, terminated by three or more diverging points. 



Cosmarium armatum, Brebisson, Menegh. Synop. Besmid. in Linncea 1840, 



p. 218. 

 Xanthidium fiircatum, Ralfs, in Annals of Nat. Hist. v. 14. p. 466. t. 12. 



f. 1 (1844), excluding synon. ; Trans, of Bot. Soc. of Edinburgh, v. 2. 



p. 154. t. 17. Jeuner, Fl. of Tunbridge Wells, p. 194. Hassall, Brit. 



Alg. p. 359. 

 Euastrum armatum, Kiitzing, Phy. Germ. p. 137 (1845). 

 Xanthidium armatum, Brebisson, in lit. (1846). 



Common. North Wales ; near Carmarthen ; and Penzance, J. R. Sussex ; 

 Kent ; Weston Bogs near Southampton, and New Forest, 3Ir. Jenner. Herts, 

 Mr. Hassall. Kerry, Mr. Andreivs. Ireland, Mr. Moore. Aberdeenshire, 

 Mr. P. Grant and Dr. Dickie. Banffshire, Mr. P. Grant. Ambleside, West- 

 moreland, Mr. Sidebotham. 



Falaise, Brebisson. Germany, K'utzing. 



This species forms, at the bottom of shallow pools, cloud-Uke masses, which 

 being detached, immediately rise to the surface. Its fronds, dispersed like 

 minute glittering dots through the connecting cloud-like substance, are visible 

 to the naked eye. 



Frond comparatively large, deeply constricted, the constriction forming a 

 linear notch on each side ; segments nearly equal in length and breadth, 

 broadest at the base, their ends rounded or trmicate ; spines short, stout, divided 

 at the apex ; of these there are generally six pairs, which are marginal, and 

 situated three on each side ; a few others usually are scattered on the disc. 



The central protuberances are cylindrical, truncate, and bordered by pearly 

 granules which produce a dentate appearance, especially in the end and lateral 

 views. The empty frond is minutely punctate. The end view is elliptic. 



I have gathered a single sporangium of Xanthidium armatum at Dolgelley ; 

 it was large and orbicular, with depressed tubercles. Having seen but this 

 example, I am unable to say whether these tubercles eventually elongate into 

 spines. 



Xanthidium Artiscon, Ehr., differs from this species not only in its smaller 

 size and its differently-shaped segments, but also in its longer spines, which 

 are nearer the ends than those of X. armatum. 



Length of frond ^^^ of an inch ; breadth -^-^ ; thickness -^^ ; breadth at 

 constriction tts-t. 



