10 BRITISH DESMIDIACEJl. 



depth of the interlobular incisions, the amount of projection 

 of the upper lateral lobes, and in the shape of the polar lobe. 

 We have given a figure of the form of this species most often 

 observed in Scotland (PI. XXXIV, fig. 1), and it will be seen 

 to differ from the figures given by Ralfs in the convex apex 

 of the polar lobe and in the less deep incisions beneath the 

 polar lobe. All intermediate stages occur between this form 

 and the one figured by Ealfs. 



Archer (in ' Quart. Journ. Micr. Sci./ 1875, p. 414) observed 

 a zygospore formed by the conjugation of E. humerosum and 

 E. Didelta. This is the only case of true hybridization ever 

 recorded among the Desmidiaceee, and it would have been 

 most interesting to have witnessed the development of the 

 zygospore. 



Some specimens possess three large scrobiculations situated 

 in the centre of the semicell between the tumours. This 

 form has been named " forma scrobiculata Nordst." 



4. Euastrum pinnatum Ralfs. 

 (PI. XXXIV, figs. 3-6.) 



Euastrum pinnatum Ealfs, Brit. Desm. 1848, p. 81, t. 13, f. 1 ; Arch, in 



Pritch. Iiifus. 1861, p. 728 ; Eabenh. Flor. Europ. Algar. Ill, 1868, p. 182 ; 



Lund. Desm. Suec. 1871, p. 19 ; Wolle, Desm. U.S. 1884, p. 98, t. 28, 



f . 14-16 ; Cooke, Brit. Desm. 1886, p. 66, t. 32, f . 2 ; De Toni, Syll. 



Alg. 1889, p. 1089 ; West, Alg. W. Ireland, 1892, p. 137 ; Alg. Eng. 



Lake Distr. 1892, p. 722 ; Roy & Biss. Scott. Desni. 1893, p. 177 ; Nordst. 



Index Desm. 1896, p. 201 ; West & G-. S. West, Alg. N. Ireland, 1902, 



p. 28. 



E. pinnatum a. typicum Eacib. Nonn. Desm. Polon. 1885, p. 93. 

 E. multilobatum Benn. Freslrw. Alg. Eng. Lake Distr. 1886, p. 8. t. 1, f. 11 



(figure bad). 

 Helierella pinnata Kuntze, Eevis. geii. plant. 1891, p. 899. 



Cells moderately large, about twice as long as broad, 

 deeply constricted, sinus narrowly linear with the 

 extremity dilated; semicells five-lobed, incisions be- 

 tween the lobes fairly deep and widely open; polar 

 lobe projecting, anvil-shaped with rounded angles, 

 apex straight, rarely retuse or convex, with a fairly 

 deep and narrow median incision ; upper lateral lobes 

 broadly mamillate, with the upper margin horizontal 

 (sometimes much broader, subquadrate, and subemar- 

 ginate) ; lower lateral lobes subquadrate, and retuse 

 with rounded angles ; semicells with a large protu- 

 berance within each of the lower lateral lobes, and 

 with three smaller ones at the base near the centre, 



