74 BRITISH DESMIDTACE.E. 



breadth without spines 40-04 //, with spines 55-8o 

 breadth of isthmus 12-20/x; thickness 31-36 /x. 



ENGLAND. Near Bowness, Westmoreland ! (Bissett). 

 Hawkshead and Hampsfell, Lancashire ! In the 

 plankton of Derwent Water, Cumberland ! 



SCOTLAND.- -Common (Hoy fy Bissett). Sutherland! 



IRELAND.- -Ballynahinch, Gralway ! Lough Gartan, 

 Donegal ! 



Geogr. Distribution.- -France. Germany. Xorway. 

 Sweden. Central China. N. India. United States. 



This distinctive variety is larger than typical X. cristatufn, 

 with more pyramidate semicells and a granulate central 

 protuberance. The spines are invariably curved, and often 

 recurved, and all the paired spines are turned in a direction 

 more nearly parallel with the longitudinal axis. 



The granulation of the central area is variable, and some- 

 times of a very irregular character. We have previously 

 figured some of these variations as observed in American 

 specimens (Me W. & G-. S. West, ' Some N. Amer. Desm.' 

 1896, fig\ 3 on p. 253). Messrs. Roy and Bissett mention a 

 curious form from Loch B-uthven, Inverness, " in which the 

 central circlet of granules is replaced by a short, stout, blunt 

 spine/' 



In many specimens the basal part of the spines is distinctly 

 dilated, and in some this character is so far extended that 

 each of the paired spines is inserted upon a mamillate pro- 

 tuberance of the cell. This form, which we have observed 

 from the United States, we have named "var. uncinatum forma 

 mucronata " (West, ' Freshw. Alg. Maine II/ 1891, p. 355, t. 

 315, f. 11). 



Var. Delpontii Roy & Biss. (PI. CXI, fig. - 



X. cristatum as described and figured by Delponte, Desm. subalp. 1877, 



p. 75, t. 14, f. 1-11. 

 X. cristatum var. Delpontei Roy & Biss. Scott. Desm. 1893, p. 244. 



Cells proportionately wider than in var. uncinatum; 

 semicells subsemicircular with stout spines ; central 

 area large and furnished with large granules, one 

 in the centre surrounded by an inner circle of 6 or 7 

 and an outer circle of 10-12. 



Length without spines 54-01 /n, with spines 80-95 p ; 



