54 BRITISH DESMIDIACE.E. 



these spines traverse tlie semicell at right-angles to the plane 

 of compression,, each series forming a short arc over one of 

 the six angles of the semicell. The variation in X. armatuin, 

 which at first sight appears very considerable, is practically 

 confined to the variation in the number, in the degree of sub- 

 division , and in the relative regularity in disposition of these 

 furcate spines. Normally, there are four such stout spines in 

 each series, but this is frequently reduced to three in some of 

 the series. The trifurcate spine also seems to be the typical 

 one, although bifurcate spines are common, and others divided 

 to a greater or lesser extent into four teeth are scarcely less 

 frequent. The large truncated wart in the centre of each 

 semicell is furnished, in the typical form, with a marginal ring* 

 of simple or emarginate teeth. The disposition of the furcate 

 spines is not always in regular series, and where such a 

 departure from the regular arrangement exists, even in only 

 a few of the series, the front view of the Desmicl presents an 

 aspect of irregularity which has been largely responsible for 

 such named varieties as " var. supernumeraria Schmidle," 

 " var. incongruum Turn.," and "var. intermedium Schroder." 



The stout spines of this species are very slightly hollowed 

 at the base, but are otherwise solid; and the punctulation of 

 the cell-wall is much more conspicuous in some individuals 

 than in others. 



We figure on Plate CY, fig*. 5, a curious form of X. armatuin, 

 which must be either a reduction- form resulting from rapid 

 division or one of the early stages in the development from 

 the zygospore. Only two specimens have been seen, both 

 from the west of Ireland in 1892. Hustedt has recently 

 mentioned (' Desm. et Bacill. aus Tirol/ 1911, p. 337, fig. 27) 

 a similar reduction-form from the Tyrol. 



Var. fissum Kbrdst. (PL CV, figs. 1, 2.) 



X. armatiim var. fissum Norclst. Alg. aq. dulc. et Char. Sand vie. 1878, p. 17. 

 t. 2, f . 6 ; Schmidle, Lappmark Siisswasseralgen, 1898, p. 21. 



Spines much more elongate, 2-3-furcate at the 

 apex; central wart of the semicells elongate and 

 similar to the marginal spines, 3-4-fnrcate at the 

 apex. 



Length (with spines) 135-1 60 /*; breadth (with 

 spines) 104-1 22 /x; breadth of isthmus 30-34 /x ; 

 thickness 80-86 . 



