76 DUBLIN NATUEAL HISTOET SOCIETY. 



page 1, January, 1861, Mr. Lobb, in his interesting paper, describing 

 the growth of the nascent segment in dividing fronds of Micrasterias, 

 throughout calls his specimens M. djenticulata, but \iq figures If. rofata. 

 Again, Dr. Wallich, in the two first — and only two as yet published — 

 of a series of papers descriptive of Desmidiacese, discovered in Bengal 

 ("Annals of ^N'atural History," 3 ser., vol. v., p. 280, 1860), affirms his 

 belief that 31. rotata and M. denticulata, met with by him very sparingly 

 in that coantry, are really but varieties of one species. But he adopts the 

 same course, indeed, with many other allied species. I am compelled, 

 however, very deferentially to differ from him. Mr. Ralfs himself, in- 

 deed, in the " British Desmidige" (p. 71), expresses " some doubt 

 whether this plant [J/, rotata] is not a variety of M. denticulata, as the 

 angles are sometimes merely acute, instead of being prolonged into teeth;" 

 but I surely think the claims of each to specific rank do not depend 

 upon this one, or upon any one character, but, as I have indicated above, 

 are founded on many and constant points of difference. It is surely 

 not an argument for their specific identity that they possess several cha- 

 racters in common — and, so far as I see, I have above indicated all such, 

 leaving out of question those that are strictly generic. Surely the ulti- 

 mate, constant, and obvious characters, possessed by one and not by the 

 other, are those upon which we must rely, and I conceive those I have 

 above poiuted out are abundantly sufficient to separate those two spe- 

 cies. It is to be conceded, indeed, that sometimes with them as with 

 other species slight variations occur, such as a greater interval between 

 the lobes, more or less acuteness of the teeth, or such like accidental 

 circumstances, but I aver that there is always a tout ensemble rendering 

 the identification a matter of no difficultj'-. I am glad to say I am sus- 

 tained in this view by that of so distinguished an authority as M. De 

 Brebisson. That naturalist, indeed, was the first to discover and name 

 M. denticulata as distinct from Euastrimi rota (Ehr.) = Micrasterias 

 rotata (Ralfs) ; and I am pleased to fi.nd that, with his accumulated ex- 

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

 me in thinking my Micrasterias Thomasiana to be a species quite distinct, 

 and very remarkable. M. de Brebisson ingeniously remarks in refe- 

 rence to 31. rotata, " Si Ton voulait exprimer le Micrasterias rotata par 

 la designation symbolique de M. Dixon, je crois qu'il faudrait aj outer 

 un lobe et dire : a, b, c, d, ' radial.' " (Por the Rev. R. V. Dixon's paper 

 alluded to, see " Natural History Review, 0. S., vol. vi., p. 464 ; Proc. 

 Nat.Hist. Soc.Dub. ;" also, "Quarterly Journal of Microscopical Science," 

 0. S. vol. viii., p. 79.) But I should not be disposed to acquiesce in this, as 

 it seems, I think, evident that the middle lobes in 31. 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 3J. 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 re- 

 present the same " subdivision" (Dixon). I have to thank M. de Bre- 



