132 BRITISH DESMIDIACE2E. 



Norclst. Desm. Arctoae, 1875, p. 33, t, 8, f. 36. Cells 

 proportionately wider; vertical view tetragonal with 

 slightly concave sides. Length 15/i; breadth 15-17/u; 

 breadth of isthmus 13 /i. (PL CXIX, fig. 4.) Hab. 

 Spitzbergen. 



Yar. CONSTRICTUM West. Constriction deeper, sinus form- 

 ing a right angle ; apex of semicells straight or slightly 

 concave. See below. 



Messrs. Roy and Bissett have recorded " St. minutissimum 

 Auersw." from Cairngorm, Inverness; Milton Moor, and 

 Tomachar in Cromar, Aberdeen; and Kerloch, Kincardine. 

 We do not know, however, to which form these records 

 refer. 



The only form we have personally observed in the British 

 Islands is the following : 



Var. constrictum West, (PL CXIX, fig. 6.) 



St. minutissimum var. constrictum West, Alg. W. Ireland, 1892, p. 172, 

 t. 24, f. 14 ; Cushman in Khodora, 1905, p. 263. 



Rather larger than the type and more deeply con- 

 stricted, apex of semicells straight or slightly concave ; 

 vertical view triangular, with snbconcave sides. 



Length 16-24/x; breadth 1 7' 5-24 /i, ;. breadth of 

 isthmus 9 f.i. 



IRELAND.- -Near Clifden, Gal way ! 



Geogr. Distribution.- -United States. 



SECTION C. 



Cells smooth or punctate, without spines, verrucae, 



or granules. 



1 Angles more or less rounded or furnished with a small solid 



mucro or mamilla. 



t Sinus open and acute-angled (or more rarely obtuse- 

 angled) . 



+ Semicells more or less elliptic, angles rounded or 

 acutely rounded. 

 Angles not thickened and without a mucro. 



6. St. muticum. 



7. St. Bieneanum. 

 S. St. ellipticinn. 

 9. St. coarctatwn. 



10. St. gr ancle. 



