216 BRITISH DESMIDIACE^. 



spines ; each semicell with 2 capitate processes of con- 

 siderable length projecting from its apex ; processes 

 disposed asymmetrically, and overlapping the adjacent 

 cell. Chloroplasts axile, one in each semicell, with a 

 central pyrenoid. Filaments long and twisted, and often 

 embedded in a mucous investment. 



Zygospores only known in one species, globose, with 

 many simple spines. 



The genus Onychonema is closely allied to Sphcerozosma, from 

 which genus it differs in the size and arrangement of its apical 

 processes. In Onychonema the two processes of each semicell 

 do not apply themselves to the processes of the neighbouring 

 semicell, but overlap it in a characteristic way, so that in the 

 front view, all the processes on the left of the filament are 

 disposed in one direction, and all those on the right in the 

 opposite direction. 



As in Sphcerozosma, Lutkemiiller (in 'Zellmembr. Desm.' 1902, 

 p. 367) has expressed his doubts that these apical processes are 

 really for the purpose of attaching the cells together, and has 

 demonstrated the presence of two cushions of jelly, closely in 

 contact, filling up the space between the cells (in Onychonema 

 filiforme), which he considered to be the real agents in effecting 

 the attachment. As in Sphcerozosma also, he suggests that their 

 real function is to counteract the tendency of the filaments to 

 twist, a tendency present in an almost dangerous degree. 



Both species of Onychonema are very rare. 



1. Onychonema filiforme (Ehr.) E. & B. 

 (PL CLX, figs. 13, 14.) 



Tessararthrafilifornns Ehr. Inf. 1838, t. 10, f. 21. 



? Odontellafiliformis Ehr. Inf. 1838, p 154. 



IsthmiafiliformisMeuegh. Synops. Desm. p. 205. 



Isthmosira filiformis Kiitz. Phyc. Germ. 1845, p. 141 ; Spec. Alg. 1849, 



p. 188. 

 Sphcerozosma filiformis ? Ealfs, Brit. Desm. 1848, p. 209 ; Arch, in Pritch. 



Inf. 1861, p. 724 ; Rabenh. Flor. Eur. Alg. 1868, p. 149 ; Arch, in Quart. 



Jourii. Micr. Sci. v. 9, 1869, p. 198 ; Lund. Desm. Suec. 1871, p. 91 ; 



Kirchn. Alg. Schles. 1878, p. 133 ; Wolle, Desm. U. S. 1884, p. 29, t. 4, 



f. 5, 6 ; Cooke, Brit. Desm. 1887, p. 5, t. 2, f. 6 ; Hansg. Prodr. Alg. 



Bohm. 1888, p. 170 ; Comere, Desm. de France, 1901, p. 199, t. 16, f. 6. 

 Onychonema Nordstedtianum Turn. New Rare Desm. 1885, p. 934, t. 15, 



f. 3 ; Cooke, Brit. Desm. 1887, p. 6, t. 2, f. 7 ; Nordst. Desm. Bornh. 



1888, p. 208 ; De Toni. Syll. Alg. 1889, p. 796 ; West, Alg. N. Wales, 



1890, p. 7 ; Turn. Alg. E. Ind. 1893, p. 139, t. 17, f. 17 ; Roy & Biss. 



