80 milTISH DESMIDIACEJE. 



strictly identical, and it is most probable that both should bo 

 merged into P. Cylmdrus. It differs from P. truncatum Breb 

 in its cell-wall and in its less truncate apices. From the 

 succeeding- species P. e.rif/ninn "West it is distinguished 

 by its different proportions, by the absence of the median con- 

 striction, by the undilated apices, and by its cell-wall. 



13. Penium exiguum West. 

 (PI. VI, figs. 0, 7.) 



Penium exiguum West, Alg. W. Ireland, 1892, p. 126, t. 19, f. 17, 18 ; 

 Nordst, Index Desm. 1890, p. 121 ; West & G. S. West, Alg. N. Ireland, 

 1902, p. 21. 



Cells small, 3-6 times longer than their diameter, 

 cylindrical, usually with a slight median constriction, 

 apices truncate and commonly slightly dilated ; cliloro- 

 plasts with 2-3 pyrenoids; cell -wall colourless, 

 delicately and irregularly punctate. 



Zygospore unknown. 



Length 18-5-37 /u; breadth G-S'5 /u. 



SCOTLAND. Near Balallan and near Barvas, Lewis ; 

 N. Uist, Outer Hebrides ! 



IEELAXD. Kylemore, Galway ! Cromagloun, Kerry! 

 Near Grlenties ; near Gweedore ; pool near Lough 

 Grlentornan. Donegal ! 



o 



This species is characterised by its relative proportions, its 

 dilated apices, and its punctate cell-wall. There is frequently 

 a terminal vacuole at each extremity of the cell, containing 

 moving granules. 



Forma major noli. (PI. VI, fig. 8.) 



Penium Leu-isii Turner, Desm. Notes, 1893, p. 346, f. 15. 



P. exiguum West, fornifx Lewisii West & G. S. West, Rec. publ. Desm. 

 1M>5, p. 00; Nordst. Index Desm. 1896, p. 122; West & G. S. West, 

 Alg. S. England, 1897, p. 478 ; Alg. N. Ireland, 1902, p. 21. 



Somewhat larger than the typical form, but other- 

 wise exactly similar. 



Length 46-62 n ; breadth 10-11 /<. 



ENGLAND. --Keighley Moor, W. Yorks ! Surrey! 

 Cornwall ! 



