o4 BRITISH DESMIDIACE^l. 



narrowing of the semicells which gives the cell a very different 

 outward form. It stands nearest in its general appearance to 

 Penium cruciferum (De Bary) Wittr., but differs in its pro- 

 portionately shorter cells, its slightly compressed vertical 

 view, and its more irregular chloroplast. 



A considerable irregularity is shown by the semicells of 

 this Desmid, the two semicells of one individual being often 

 of different form. The sinus varies mnch in shape, and the 

 apex of the semicell may or may not be subtrnncate. 



A rather narrower form of this species, with cells almost 

 twice as long as broad, occurs in the plankton of Loch Asta, 

 Shetlands. Length 17 j*; breadth 9'2 fi. (PL LXVIII, fig. 15.) 



67. Cosmarium pericymatium Xordst. 

 (PL LXVIII, fig. 10.) 



Cosmarium pericymatium Nordst. Desni. Arctoa?, 1875, p. 29, t. 7, f . 26; 



De Toni, Syll. Alg. 1889, p. 1038; Johnson, Rare Desni. U.S. II, 1895, 



p. 293, t. 240, f. 28. 



Ursinella pericymatia Kuntze, Revis. gen. plant. 1891, p. 925. 

 Dyspliinctium pericymatium Schniidle, Beitr. alp. Alg. 1895, p. 348. 



Cells of moderate size, about 1-g- times longer than 

 broad, moderately constricted, sinus open and obtuse ; 

 semicells semi-elliptic with a slightly contracted base, 

 margin gently undulate with about 16 small undula- 

 tions. Side view of semicell semi-oblong-elliptic with 

 a very slightly contracted base. Vertical view very 

 broadly elliptic, slightly produced at the poles. Cell- 

 wall somewhat thick and densely punctate. 



Zygospore unknown. 



Length 40-51 p.; breadth 28-32 /*; breadth of 

 isthmus 23-25 p ; thickness 24-27 p. 



Geogr. Distribution. Germany. Sweden. Spits- 

 bergen. United States. 



The typical plant is not known to occur in the British 

 Islands. 



Var. eboracense West & G. S. West. (PI. LXVIII, 



fig. 11.) 



G. pericymatium var. eboracense West & G. S. West, Notes Alg. II, 1900, 

 p. 292, t, 412, f. 4 ; Alga-fl. Yorks. 1900, p. 88. 



A smaller variety ; semicells with fewer and more 

 prominent undulations. 



