188 BRITISH DESMTDIACEJE. 



has been botamcally investigated. As would be expected 

 from its cosmopolitan distribution, it exhibits a considerable 

 amount of variation, and some fifteen named varieties have 

 been described by different authors. 



In its most typical form the basal angles of the semicells 

 are rectangular, and the upper portions of the sides are 

 straight and rapidly converging* to a somewhat narrow, sub- 

 truncate apex. The sides are,, however, frequently concave 

 in their upper portions, this form having been named "var. 

 cuncavum r by Lagerheim. As there is every intermediate 

 condition we do not think the forms with the concave sides 

 constitute a variety worthy of a special name. 



Another form, which is sometimes met with abundantly, 

 possesses slightly inflated semicells, so that the sides are 

 distinctly convex and the apex is not so evidently subtrun- 

 cate. This form has been figured by Gntwinski (' Xagj. 

 dosel. Bosni Hercegovin. halugain/ 1896, p. 374, t. 1, f. 2 a') 

 and by Borge (' Subfoss. sotv. alg. Got!/ 1892, t. 1, f. 4). 



Var. subgranatum Nordst. (PL LXIII, figs. 5-8.) 



C. granatum Breb. var. subgranatum Nordst. Alg. aq. cUilc. et Char. 

 Sanclvic. 1878, p. 13, t. 2, f . 8 ; De Toni, Syll. Alg. 1889, p. 932 ; West, 

 Alg-. W. Ireland, 1892, p. 143 ; in Naturalist, 1893, p. 213 ; Johns. Rare 

 Desm. U.S. II, 1895, p. 293, t. 239, f. 16 ; Nordst. Index Desm. 1896, 

 p. 135 ; West & G-. S. West, Alga-fl. Yorks. 1900, p. 79 ; Alg. N. Ireland, 

 1902, p. 32. 



Semicells near the base at first diverging 1 for a short 



o o 



distance, then with the lateral margins converging and 

 12 -undulate, apex very narrowly truncate ; vertical 

 view narrowly elliptic and slightly tumid at the middle 

 on each side. 



Zygospore globose, furnished with a few short simple 

 spines, each spine arising from the apex of a depressed- 

 conical projection. 



Length 24-30 p.; breadth 17-22 /x; breadth of isth- 

 mus 6-9 /x; thickness 13-14 jit; diam. zygosp. without 

 spines 25-29 /x, with spines 32-34*5 p.. 



ENGLAND.- -Westmoreland ! W., N., and E. Yorks ! 

 Lincolnshire ! Essex ! Cambridgeshire ! Gloucester 

 (with zygospores from near Cirencester) ! Surrey ! 

 Devon ! Cornwall ! 



SCOTLAND. Laro;s and Cumbrae, Ayrshire ! Suther- 



< j ' 



