88 BRITISH DESMIDIACE.*:. 



Another Desmid described by Kaciborski (' Desm. Nowe/ 

 1S89, p. 85, t. 5, f. 29), as C. suborthogonum, only differs from 

 C. impressulum in the presence of a slight protuberance in 

 the middle of each side of the vertical view, and this form 

 would in consequence be best placed as C. impressulum forma 

 subortho'jona. 



106. Cosmarium umbilicatum Liitkem. 

 (PI. LXXII, figs. 19-21.) 



Cosmarium umbilicatum Liitkem. Desm. Attersees, 1893, p. 550, t. 8, f . 2 ; 

 Nordst. Index Desm. 1896, p. 265 ; West & Gr. S. West, Alg. S. England, 

 1897, p. 487 ; Alga-fl. Yorks. 1900, p. 92. 



Cells small, a little longer than broad, deeply con- 

 stricted, sinus very narrow with a dilated apex ; semi- 

 cells angularly subsemicircular, sides triundulate, 

 lower parts of sides upwardly diverging or rarely sub- 

 parallel, upper parts strongly converging, upper and 

 lower angles slightly rounded, apex truncate and 

 straight, with a uniscrobiculate tumour in the centre 

 of each semicell. Side view of semicell subcircular. 

 Vertical view elliptic, with a slight protuberance at 

 the middle on each side. Cell-wall finely punctate. 



Zygospore unknown. 



Length 18-20 ^ ; breadth 15'5-17/x; breadth of 

 isthmus 5-5*5 /i ; thickness 10' 5 JJL. 



ENGLAND.- -Malton, X. Yorks! Brent Reservoir, 

 Middlesex ! 



Geor/r. Distribution.- -Austria. 



This small species stands nearest to C. impressulum forma 

 suborthogona and C. perpusillum. 



107. Cosmarium perpusillum West. 

 (PI. LXXII, figs. 22, 23.) 



Cosr.w.-iuni perpusillum West, Alg. W. Ireland, 1892, p. 148, t. 21, f. 2; 

 Nordst. Index Desm. 1896, p. 199. 



Cells minute, a little longer than broad, very deeply 

 constricted, sinus narrowly linear with a dilated apex ; 

 semicells subhexagonal, angles very slightly rounded, 

 lower lateral margins slightly retuse, upper lateral 



