246 ARCHEB, ON A NEW SPECIES OF MICBASTERIAS. 



draw attention to the distinctions between them. M. Thoma- 

 siana is notably smaller than M. denticulata (as it is smaller 

 than M. rotata) ; in the new species the basal and middle 

 lobes are mostly thrice dichotomous — in M. denticulata they 

 are but twice dichotomous ; in the former^ the ultimate sub- 

 divisions of the basal and middle lobes^ when eighty are biden- 

 tatCj when four only^ tridentatCj — in the latter they are four 

 only, and truncato-emarginate, with rounded, or sometimes 

 subacute, angles ; in' the former, the terminal emargination 

 of the end lobe is rather deep and acute — in the latter it is 

 more shallow and more rounded. To compare M. Tliomas'iana 

 with M. rotata, the former is much smaller than the latter 

 (which, indeed, is larger than M, denticulata) ; in the former 

 the basal and middle lobes are mostly thrice, sometimes twice, 

 dichomotous — in the latter the basal lobes are constantly 

 twice, the middle lobes thrice, dichomotous; in the former 

 the teeth of the ultimate subdivisions are not so lengthened 

 as in the latter ; in the former the middle lobe is narrow, and 

 with nearly parallel sides below, widening above — in the 

 latter the end lobe is more or less campanulate ; in the former 

 the end lobe is wholly included, its angles acute, the central 

 notch deep, acute — in the latter, the end lobe is slightly but 

 distinctly exserted, its angles somewhat produced and biden- 

 tate, its central notch shallow and rounded. But, above all, 

 the new species is remarkably distinct from both M. denticu- 

 lata and M. rotata, and, indeed, every other species, by the 

 striking projections at the base of the segments, and by the 

 superficial eminences. No distinctions can be drawn from 

 the sporangium in these species, as, unfortunately, it is un- 

 known, that of M. denticulata excepted. It does not appear 

 at all requisite to compare M. Thomaslana with any other 

 species, as there is none other for which there seems any 

 chance of its being mistaken. Indeed, the only other species 

 of Micrasterias with which I am acquainted that possess any 

 processes or projections directed in a different plane from that 

 of the frond, are M. muricata, Ralfs, and M. Americana, 

 Ralfs. In those species, however, the projections are at the 

 external margin of the segment, not at their base ; and the 

 entire form and structure and appearance of the frond is 

 wholly and completely different, any characters in common 

 being, of course, those only of generic value. 



Some may think, I fear, that I have gone to unnecessary 

 length in carrying out the intention of this paper. I can 

 only apologise by saying 1 am anxious that these common 

 species of Micrasterias, M, rotata and M. denticulata, remark- 

 ably beautiful and favorite microscopic objects, should be 



