ARCHER^ ON A XE^Y SPECIES OF MICRASTERIAS. 245 



guished an authority as M. de Brebisson. That naturalist^ 

 indeed^ was the first to discover and name M. denticulata as 

 distinct from Euastrum rota, 'E\ir. = 3I. I'otata, Ralfs ; and 

 I am pleased to find that^ Avith his accumulated experience, 

 he still thinks them good species, and, moreover, concurs 

 with me in thinking my M. Thomasiana to be a species quite 

 distinct, and very remarkable. M. de Brebisson ingeniously 

 remarks, in reference to M. rotata, '' Si I'on voulait exprimer 

 le M. rotata par la designation symbolique de M. Dixon, je 

 crois qu^il faudrait aj outer un lobe et dire : a, b, c, d, 'radial/ ""^ 

 But I should not be disposed to acquiesce in this, as it seems, 

 I think, evident that the middle lobes in M. rotata have the 

 primary incision merely very deeply carried down, and that 

 the primary dichotomous subdivisions cannot be said to 

 represent two lobes ; this portion in this species and that in 

 M. denticulata, in which the external (middle) lobes are only 

 twice dichotomous, are homologous (if that term can be 

 applied to an unicellular plant without special organs), and 

 represent the same '^ subdivision" (Dixon). I have to thank 

 j\I. de Brebisson^s kindness for dried specimens of each of 

 those species from France, and they appear to me to be quite 

 identical with our own.f 



I now come briefly to contrast my new species with M. 

 rotata and M. denticulata ; and, as in the instances of those 

 two species, it will be better first to point out where the 

 new species agrees Avith them ; but as that is, indeed, pre- 

 cisely in the characters in Avhich they agree with one another 

 as detailed above, it will be, therefore, quite unnecessary 

 again to narrate those characters (vide supra). I say M. 

 Thomasiana agrees with all the common positive and nega- 

 tive characters of M. rotata and M. denticulata given aboA'c, 

 for the superficial apiculate projections on the former 

 cannot be called either papillae or granules ; they are the 

 summits of the eminences disposed over the frond. The 

 former seems to me to have a greater affinity Avith M. denti- 

 culata than Avith M. rotata ; indeed, in front view there is a 

 liability of the former being confounded Avith it. I shall first 



* 'Natural History Review,' O. S., vol. vi, p. iQi; 'Proc. Nat. Hist. 

 Soc. Dab.'; also 'Quarterly Journal of Microscopical Science,' 0. S., vol. 

 viii, p. 79, "On a New Genus and Species in tlie Desmidiacese, with some 

 remarks on tiie Arrangement of the Genera and Species of Micrasterias and 

 Euastrum," by the Rev. R. V. Dixon, A.M., Ex. E.T.C.D. 



f Since this paper was read I have had an opportunity, through the 

 kindness of my friend, Mr. W. Keay, of examining a gathering made near 

 Dundee, in which 31. denticulata occurred, and in no point did his specimens 

 differ from that species collected in this country ; and the same might be 

 said of the other forms therein common to Scotland and Ireland. — W. A. 



