*J0 BRITISH DESMIDIKiE. 



them as distinct, has not separated them from the other species of 

 Micrasterias. 



* Frond circular ; segments five-lobed ; lobes approximate, the end 



lobe narrow. 



1. M. denticulata (Br^b.) ; frond orbicular, smooth; segments five- 

 lobed ; lobes dichotomously divided, the ultimate subdivisions 

 truncato-emarginate with rounded angles. 



Micrasterias denticulata, Brebisson, Alg, Fat. p. 54. t. 8. (1835). 

 Euastrum Rota, Elirenberg, Infus. t. 12. f. 1. «. (1838). Bailey, Amer. Bacil. 



in Amer. Journal of Science and Arts, v. 41. t. 1. f. 22. 

 Micrasterias Rota, Meneghini, Syn. Desmid. in Linncea 1840, p. 215. 

 Micrasterias rot at a, Ralfs, Annals of Nat. Hist. v. 14. t. 6. f. 1. (1844) ; 



Trans, of Bot. Society of Edinburgh, v. 2. t. 10. 



Common. Wales ; Penzance, &c.,J.R. Barmouth, Rev. T. Sahoey. Near 

 Soi;thampton ; Sussex and Kent, Mr. Jenner. Henfield, Mr. Borrer. Ste- 

 venston, Ayrshire, Rev. D. Bandshorough. Yate near Bristol, Mr. Broome. 

 Ambleside, Westmoreland, Mr. Sidebotham. Aberdeenshire, Dr. Dickie and 

 Mr. P. Grant. 



Falaise, Brebisson. Germany, Ehrenberg . West Point, New York, Bailey. 



The frond is large and circular ; each segment is five-lobed. The end lobe 

 is the narrowest, is simply emarginate, and often appears more turgid than the 

 others. The lateral lobes are dichotomously incised, their ultimate sub- 

 divisions truncato-emarginate. The lobes and subdivisions are alike cuneate, 

 approximate, and radiating from the centre. The endochrome is bright green, 

 and the starch vesicles scattered and conspicuous. Frequently the margin of 

 the frond is colourless. 



I once gathered sporangia of this species rather plentifully in a boggy pool 

 near Dolgelley. Tbe lenticular shape of the fronds and their conjugation in a 

 crossed position render it almost impossible to obtain a satisfactory view 

 without displacing them. The process appears similar to what takes place in 

 Staurastrum dejectmn ; the contents of both fronds unite and form a globular 

 sporangium between them, the connexion between this sporangium and the 

 empty segments being ascertained merely by their contiguity and by their 

 relative position, which undergoes no change during the movements of the 

 sporangium. The sporangium is inclosed in a fine membrane, and its surface 

 is gradually furnished with scattered, stout, elongated spines, which are at 

 first simple and their apex obtuse, but this afterwards becomes forked or trifid, 

 and its divisions finally are again branched and frequently more or less re- 

 curved. The sporangia are of considerable size, — a necessary consequence of 

 the union of the contents of both fronds. 



The truncate ends of the subdivisions distinguish this species from Micra- 

 sterias rotata, to which it is very closely allied. 



Length of frond yy^ of an inch ; breadth of segment ~-^ ; breadth at con- 



