COSMARIUM. 265 



ENGLAND.- -Ingleton, W. Yorks (on dripping lime- 

 stone rocks) ! 



Geoyr. Distribution.- -Germany. Bohemia in Austria. 

 Spitzbergen. China. 



C. tumens is apparently a very rare Desmid with most 

 characteristic features. Its inflated semicells, open sinus, and 

 peculiarities of granulation easily distinguish it from any 

 other of the crenulated species. 



221. Cosmarium retusum (Perty) Rabenh. 

 (PI. XCI, figs. 1, 2.) 



Euastrum (Cosmarium') retusum Perty in Mittheil. naturf. Ges. Bern. 



1849, p. 173 ^Not E, retusum Kiitz., 1845 = Arthrodesmus Incus (Breb.) 



Hass." ; Kleinst. Lebensf. 1852, p. 208, t. 16, f. 12. 

 Cosmarium retusum (Perty) Eabenh. Flor. Europ. Alg. Ill, 1868, p. 167 ; 



Lund. Desm. Siiec. 1871, p. 36, t. 3, f . 3 ; Nordst. Norges Desm. 1873, 



p. 16; Jacobs. Desm. Danemark, 1876, p. 195; De Toni, Syll. Alg. 1889, 



p. 1003; Eacib. Desmidyja Ciastonia, 1892, p. 377; Eoy & Biss. Scott. 



Desm. 1894, p. 174; Nordst. Index Desm. 1896, p. 224; West & G. S. 



West, Welw. Afric. Fresliw. Alg. 1897, p. 113. 

 Ursinella retusa Kuntze, Eevis. gen. plant. 1891, p. 925. 



Cells small, about 1-J- times as long as broad, very 

 deeply constricted, sinus narrowly linear, with a slightly 

 dilated apex ; semicells subtriangular from a subreni- 

 form base, basal angles broadly rounded, sides slightly 

 concave, apex subtruncate (very slightly convex), 

 apical angles obtuse ; cell-wall indistinctly granulate, 

 granules rather acute, very localized and variable, 6-8 

 at each basal angle (about 3 marginal), 3-6 in the centre 

 (variably disposed), and sometimes with 3 or 4 imme- 

 diately within the apex, between the granules minutely 

 punctate. Side view of semicell ovate, apex sub- 

 truncate. Vertical view rather narrowly elliptic, with 

 two (or three) slight prominences at the middle on each 

 side, each prominence tipped with a granule. Chloro- 

 plasts axile, with a central pyrenoid. 



Zygospore unknown. 



Length 30-39 p ; breadth 24-30 p, ; breadth of 

 isthmus 8-10 /x : breadth of apex 13-18 /x ; thickness 

 13-5-18-5 UL. 



