MICKASTERIAS. 115 



the identity of any of the known species of Micrasterias, but, 

 fortunately, some of Agardli's original specimens have been 

 examined by Xordstedt, and a drawing of one of them by 

 Turner, These have been shown beyond a doubt to be 

 M. rotata (Grev.) Ralfs, and J/. denticulata Breb., two species 

 which were not differentiated until many years after the 

 publication of Agardh's J/. furcata. Thus, Ralfs was greatly 

 in error when he referred J/. radiata Hass. to M. furcata Ag., 

 and J/. radiata Hass. remains as the first name given to the 

 characteristic Micrasterias which has been generally referred 

 to either as " J\I. furcata AST." or " M. furcata Ralfs." 



/O f 



J/. radiata Hass. is more variable in the subdivision of the 

 lateral lobes than any other species of the genus. This 

 was admirably shown by Johnson (' Bot. Gazette/ 1894, 

 pp. 58-60, pi. vi, figs. 7-14), and after examining* numerous 

 American forms of this species, we thoroughly agree with him 

 that J/. pseudofurcata Wolle, M. furcata var. decivrta Turn., 

 and M. furcata var. simplex, Wolle, are merely forms of 

 M. radiata between which it is impossible to discriminate. It 

 is not uncommon to find that one semicell represents typical 

 M. radiata and the other semicell of the same plant represents 

 M. pseudofurcata. This variation is of far commoner occur- 

 rence in the United States than in the British Islands, possibly 

 because JIT. radiata is much more generally distributed in the 

 United States than in this country. In Britain we have only 

 met with this notable variation of the lateral lobes in the 

 Scottish plankton, and it is possible that the variation is 

 largely due to rapidity of cell-division. 



Forms are occasionally met with in which the superior 

 lateral lobules (or even the superior lateral lobes) are up- 

 wardly curved towards the processes of the polar lobe (PL LIT, 

 fig*. 6). Similar forms have also been seen from America 

 (vide W. & GL S. West, ' Some Desm. of the U.S.' 1898, 

 p. 295, fig. xylogr. 2 ). 



J/. radiata should never be confused with M. Crux-meli- 

 tensis, a species to which it only bears a superficial resem- 

 blance. The tabulation is deeper, and the lobules are much 

 more elongate and bifurcate. It is also a larger species. 



In some specimens the lobules are of great length, approach- 

 ing a plant described by Wolle as M. dichotoma. We give a 

 figure of one of these forms (PI. LII, fig. 7). 



The lateral lobules are sometimes entire and apiculate, and 

 occasionally they are furcate in a plane at right angles to the 

 principal plane of the cell (vide PL LII, figs. 8 and 9). 



