80 NATTTEAL HISTOEY SOCIETY OP DUBLIN. 



who examines them. Dr. Wallich himself, indeed, in describing his 

 form more at large, points out briefly its distinctive marks ; but then he 

 considers these as only of importance sufficient to induce him to regard 

 the form in question as a variety of M. Americana. 



But I must urge that if, in distinguishing species in this family, dif- 

 ferences, — tangible, readily describable, constant (at least in British 

 species, even if often less striking, I believe them on the whole to be 

 constant) characteristic differences are to be lost sight of and ignored, 

 and only resemblances to be sought for and taken into account, I do not see 

 any limit to the doing away with any number of species, nor to the con- 

 solidation of perhaps a whole genus, or even of groups of genera, as single 

 species. Imbued, however, with the views which he holds as regards 

 the Desmidiacege, Dr. Wallich considers that the twenty (and upwards) 

 species belonging to the genus Micrasterias " are reducible to less than 

 half that number without infringing on a single reliable distinction." 

 From this opinion I venture, indeed, to dissent. So far as I can see, I 

 do not think it is possible to reduce their number in our books by more 

 than one or two. Parenthetically I may here, perhaps, just mention 

 those cases. I allude to Micrasterias Crux-Melitensis (Ralfs), and M. 

 furcata (Ag.). The former only of these have I myself seen ; but before 

 I read Dr. Wallich' s statement of having met in India " every state 

 intermediate" between these two, I was disposed to suspect their actual 

 difference. Now, it is to be noted that Dr. Wallich does not in any 

 other instance, while grouping together forms under a common specific 

 designation, speak of " every state intermediate." The other instance 

 may be M. truncata (Breb.) and M. crenata (Breb.). With these pos- 

 sible exceptions, I do not, at least at present, see the smallest reason to 

 interfere with the established species in this genus. 



The consolidation and combining of well-marked species would 

 appear to me quite as much to be deprecated on the one hand, as the 

 wholesale making of species on accidental differences on the other. Far 

 be it from me to aver that in this family all the individuals of any 

 certain species seem to be, as it were, cast in the same mould. Even in 

 the very form in question {M. Americana), I have noticed sometimes 

 one of the segments without the vertically-set processes of the end lobe — in 

 fact, like the figure in Ralfs.* The segment wanting these processes may 

 have been the younger, and not fully grown ; I have not seen a frond in 

 which they were wholly absent on both segments. I have met M. denticu- 

 lata with an almost entire semi-orbicular segment, something like that of 

 Cosmarium Ralfsii, and again one segment somewhat like a nondescript 

 Euastrum. Again, Docidium Ehrenbergii sometimes presents a contract- 

 ed inflated segment, somewhat like a nondescript truncate Cosmarium. 

 I have seen a Euastrum with one segment as it were double, giving the 

 entire a somewhat Y-shape ; and other examples might be cited. Can 

 it be assumed that any such is likely at the next repeated process of 



* Op. cit. t. x., 1 d. 



