10 BRITISH DESMIDIACEJE. 



C. venustum is a characteristic species which exhibits some 

 variation in the relative prominence of the lateral undula- 

 tions, in the degree of rotundity of the basal angles of the 

 semicells, and in the width of the isthmus. It occurs most 

 abundantly in the upland Sphagnum-pools. Nordstedt has 

 described and figured a trigonal variety of it (" ]3 trigonum ") 

 from Russian Lapland (vide ' Desmid. Arctose/ 1875, p. 41, 

 t. 8, f. 42). 



It is impossible to draw a distinction between C. venustum 

 and C. Gambricum, as the undulation of the sides of the 

 semicells varies much in specimens even from the same 

 locality. 



Forma minor AVille. (PL LXVI, fig. 4.) 



C. venustum ? forma minor Wille, Ferskv. Alg. Nov. Senilj. 1879, p. 43 ; 

 Boldt, Siber. Chlorophy. 1885, p. 104, t. 5, f . 10 ; West, Alg. TV. Ireland, 



1892, p. 147 ; Schmidle, Alg. aus Sumatra, 1895, p. 302 ; West & G. S. 

 West, Alg. N. Ireland, 1902, p. 35. 



C. Cambricum Cooke & Wills forma minor Turn. Freshw. Alg. E. India, 



1893, p. 70, t. 10, f. 15. 



About two-thirds the size of the type. 



Length 20-25,*; breadth 16-18/t; breadth of 

 isthmus 6-7 /u. 



SCOTLAND.- -Ben Lawers, Perth! 



IKELAND.- -Ballynahinch and Athry Lough, Galway ! 

 Gortahork, Donegal ! 



Geogr. Distribution.--GrGTiD.aiij. Nova Zembla. Si- 

 beria. India. Sumatra. 



Lundell (' Desm. Suec/ 1871,, p. 23) records the occurrence 

 of a fragile chain of seven individuals of a small form of 

 C. venustum. 



There is little doubt that C. Cambricum var. dubium Eacib. 

 ('Nonn. Desm. Polon/ 1885, p. 79, t, 11, f. 10) is a minute 

 form of C. venustum which is only about half the size of 



V 



Wille's forma minor. 



Var. hypohexagonum West. (PI. LXVI, figs. 5, 6.) 



C. venustum var. liypoliexagonum West, Alg. TV. Ireland, 1892, p. 147, 



t. 21, f. 1 ; Alg. Engl. Lake Distr. 1892, p. 726. 

 C. venustum var. hypohexagonum forma incrassata West, Alg. W. Ireland, 



1892, p. 147, t. 24, f. 23. 



Sides of semicells tricrenate rather than triundulate, 



