172 BKITISH DESM1DIACE.E. 



around the angles, those at the apices having no regular dis- 

 position. The alternation of the angles of one semicell with 

 those of the other is of no specific importance, notwithstand- 

 ing the fact that the specific name was based upon this 

 character. Such alternation is common in Staurastra of this 

 kind, and is frequently observed in St. punctulatum and St. 

 dilatatum. 



The zygospore has been described by Ralt's, but not 

 figured. We are not sure, however, that the zygospore 

 described by Ralfs belonged to St. alternans, as for many 

 years Ralfs was greatly confused between this species and St. 

 hexacerum ( = St. tricorne Ralfs). We have ourselves once 

 observed a zygospore (from Thursley Common, Surrey), but 

 the specimen was lost in attempting to permanently mount it. 

 Unfortunately this happened before a drawing had been made. 



We regard St. alternans var. coronatum Schmidle (1895) 

 as a variety of St. punctulatum Breb. 



Var. pulchrum Wille. (PL CXXVII, fig. 10.) 



St. alternans var. pulchrum Wille, Ferskv. Alg. Nov. Seinlj. 1879, p. 53 

 t. 13, f. 66; Boldt, Siber. Chlorophy. 1885, p. 115; Desm. Gronland, 

 1888, p. 37 ; Roy & Biss. Scott. Desm. 1894, p. 16 (sep.) 



Cells proportionately a little longer, with an obtuse 

 sinus ; lower lateral margins of semicells slightly con- 

 cave, apex less convex. 



Length 28-8-36/i; breadth 22 '8-33 '6^; breadth of 

 isthmus 7'2-12/x. 



SCOTLAND.- -Glen Garry near Dalnacardoch, Perth ! 



Geogr. Distribution. - Nova Zembla. Greenland. 

 Siberia. 



This variety approaches St. punctulatum, but differs in the 

 sinus, the form of the semicells, and the blunter angles 

 (especially in the vertical view). 



32. Staurastrum dilatatum Ehrenb. 

 (PI. CXXVI, figs. 10-15.) 



Staurastrum dilatatum Ehrenb. Infus. 1838, p. 143, t. 10, f. xiii [descrip- 

 tion and figures very poor] ; Ralfs in Ann. Mag-. Nat. Hist, xv, 1845, 

 p. 156, t. 1 1, f. 5 ; Brit, Desm. 1848, p. 133, t. 21, f. 8 [figures not good] ; 

 Arch, in Pritch. Infus. 1861, p. 741 ; Eabenh. Flor. Europ. Alg. Ill, 

 1868, p. 207 ; Kirchn. Alg. Schles. 1878, p. 165 ; ? ? Wolle, Desm. TJ. S. 

 1884, p. 128, t. 52, f . 32, 33 ; Nordst. Desm. Gronland, 1885, p. 10 ; Cooke, 

 Brit. Desm. 1887, p. 162, t. 54, f. 8 [figures bad] ; Hansg. Prodr. Algenfl. 



