STAURASTRUM. 49 



quently emarginate or compound ; lateral margins 

 provided with a series of spines extending from the 

 angle of the truncate apex nearly to the sinus, and with 

 a few spines just within each angle. Vertical view with 

 the lateral margins distinctlv concave, angles acute, 



*j O 



terminating in a sharp spine, and surrounded by one 

 or two concentric series of spines, with a pair of spines 

 at the base of each angle ; centre of apex smooth. 

 Spines often becoming emarginate. 



Zygospore unknown. 



Length about 44 u. ; breadth about 52 u ; breadth 

 of isthmus 25 ^ . 



ENGLAND.--Bisley Bog, Lancashire (Roy). Mickle 

 Fell, N. Yorks ! Enbridge Lake and Woolton Pond, 

 Hants (Roy). Dartmoor, Devon (Harris). 



WALES.- -Bettws-y-coed (Roy) and Capel Curig !, Car- 

 narvonshire. 



SCOTLAND. --Xear Brin, Inverness ; near Cambus 

 O'May and Tomachar, Aberdeen ; near Loch Clunie, 

 Perth (Roy & Biss). 



IRELAND.- -Dublin and Wicklow (Arch.). 



Geogr. Distribution.- -France. Switzerland. G-alicia 

 in Austria. Xorway. Spitzbergen. Greenland (form). 



St. oligacanthum is very closely allied to the preceding species, 

 with which it has probably been frequently confused by many 

 workers. As Archer (in ' Q. J. M. S.' vol. 6, 1866, p. 189) 

 points out, there are, however, distinct differences. In St. 

 cristatum the semicells are elliptic fusiform, both ventral and 

 dorsal margins are convex, and there is a mucronate angle on 

 each side (usually submamillate). In St. oligacanthum the 

 semicells are angular and roughly hexagonal with a markedly 

 truncate apex. In the vertical view the lateral margins of 

 St. cristatum are slightly convex, but in St. oligacanthum distinctly 

 concave. 



Var. incisum West. (PL CXXXIX, fig. 7.) 



Staurastrum oligacanthum var. incisum West, Alg. W. Ireland, 1892, p. 173, 

 t. 22, f. 17. 



Semicells with a distinct incision in the lower lateral 

 margins ; vertical view triangular with slightly convex 

 margins. 



VOL. v. 4 



