COSMAEIITM. 147 



We have only observed this large form from the west of 

 Ireland. The broad isthmus, slightly rounded basal angles, 

 and the more irregular disposition of the granules are its 

 principal features. As in the type the vertical view is almost 

 circular. 



The Desmid described as " C. isthmium West var. horizon- 

 tale Schmidle' ('Lappmark Susswasseralgen/ 1898, p. 31, 

 t. 1, f. 40) is most likely a form of Cosmarium subexcavatum 

 with regularly disposed granules. 



147. Cosmarium excavatum Nordst. 

 (PL LXXVII, fig. 12.) 



Cosmarium excavatum Nordst. Desm. Brasil. 1870, p. 214, t. 3, f. 25 ; Wille, 

 Ferskv. Alg. Nov. Semlj. 1879, p. 47 ; Roy & Biss. Scott. Desm. 1894, 

 p. 101 ; Nordst. Index Desm. 1896, p. 121. 



Dysphinctium? excavatum (Nordst.) De Toui, Syll. Alg. 1889, p. 884. 



Cells small, almost twice as long as broad, slightly 

 constricted, sinus a very wide and shallow depression, 

 isthmus much elongated ; semicells subsemicircular, 

 cell- wall granulate, margin about 9-granulate (or sub- 

 undulate), within the margin with rather few and 

 somewhat sparsely (subconcentrically) disposed 

 granules. Vertical view circular, granulate, with 

 about 17 peripheral granules. 



Zygospore unknown. 



Length 19 ,u ; breadth lO-10'o /u ; breadth of 

 isthmus 7*5 /n. 



SCOTLAND.- -Aberdeen, Kincardine, Forfar (Roy fy 

 Bissett) . 



IRELAND . Dublin and Wicklow (Archer). 



Geo g r. Dis tribi 1 1 to n. Norway . S weden . Nova 

 Zembla. Greenland. Japan. New Zealand. United 

 States. Brazil. Paraguay. 



o / 



We have never examined any Desmid which we could refer 

 to the typical form of this species, aud we suspect that the 

 records of Messrs. Eoy and Bissett really refer either to the 

 " forma duplo-major " or to C. isthmium. 



C. excavatum Nordst. var. trigonum Lagerh., described 

 from Georgia, U.S.A. (vide Lagerh. Biclr. Amerik. Desm.-fl. 

 1885, p. 236, t. 27, f. 7), is very possibly a species of the genus 

 Staurastrum. 



