118 BEIT1SH DESMIDIACEJE. 



Micrasterias Americana (Ehrenb.) Ealfs, Brit. Desm. 1848, p. xix; Arch, 

 in Pritch. Infus. 1861, p. 726, t. 2, f. 44 ; Eabeiih. Flor. Europ. Algar. 

 Ill, 1868, p. 189 ; Kirclm. Alg. Scliles. 1878, p. 161 ; Wills in Midland 

 Naturalist, iv, 1881, t. 5, f . 4 : Wolle, Desm. U.S. 1884, p. 112, t. 32, f . 2 ; 

 Cooke, Brit. Desm. 1886, p. 56, t. 30, f. 1 ; Mask. Note Micr. Americana, 

 1888, p. 9 ; De Toni, Syll. Alg. 1889, p. 1134 ; West, Alg. N. Yorks. 1889, 

 p. 291 ; Alg. N. Wales, 1890, p. 286 : Alg. W. Ireland, 1892, p. 134 ; Alg. 

 Eng. Lake Distr. 1892, p. 722 ; Eoy & Biss. Scott. Desm. 1893, p. 173 ; 

 Nordst. Index Desm. 1896, p. 42 ; West & Gr. S. West, Alg. S. England, 

 1897, p. 484 ; Alga-fl. Yorks. 1900, p. 67 ; Scott. Freshw. Plankton, I, 

 1903, p. 526. 



M. morsa Ealfs, Brit. Desm. 1848, p. 74, t. 10, f. 1. Ealfs in his ' Errata,' 

 p. xix, states "for H. morsa read M. Americana (Ehr.)."] 



Didymidium (Micrasterias) morsum Eeinsch, Algeiin. Frank. 1867, p. 143. 



Micrasterias Americana f. genuina Mask. Note Micr. Americana, 1888, t. 1, 



f. 1. 

 M. Americana var. typica Eacib. Desm. No\ve, 1889, p. 106 ; Gntw. Flor, 



G-lon. Okolic Lwowa, 1891, p. 74. 

 Helierella Americana Kimtze, Eevis. gen. plant. 1891, p. 898. 



Cells of medium size, about 1^- times longer than 

 broad, subhexagonal, very deeply constricted, sinus 

 somewhat open, towards the apex acute ; semicells 

 5-lobed (sometimes almost 3-lobed owing to shallow 

 incision between the lateral lobes) ; polar lobe large, 

 broad, and outstanding, cuneate from the base upwards 

 (rarely sub quadrate in the basal part), apical margin 

 widely retuse, angles each produced into a thick diver- 

 gent process with a truncate-denticulate apex, from 

 near the base of each process a second accessory pro- 

 cess arises, similar but smaller, and the two accessory 

 processes are disposed asymmetrically one on each 

 side of the polar lobe ; incisions on each side of polar 

 lobe widely open; lateral lobes rather small, incision 

 between them open and not very deep, the two lobes of 

 each side being together trapezoid in form, each lobe 

 divided into two lobules by a wide and shallow notch, 

 the two proximate lobules being smaller than the two 

 distal lobules, lobules with three or four teeth at the ex- 

 tremitv and dentate alono- their inner margins. Semi- 



/ O O 



cells with a few more or less scattered granules arranged 

 on a small protuberance in the middle above the isthmus, 

 with about four very large, acute granules within the 

 apical margin of the polar lobe, and with a number of 

 scattered smaller granules on the surface of both polar 



