202 DUBLIIT NATTJEAL HI8T0BT SOCTHTT. 



Mr. W. Abcheb, Secretary, next read the following paper — 



Ojr A NEW GENU8 AND SPECIES IN THE DESMIDIACKSi; WITH SOME REMARKS 

 ON THE ARRANGEMENT OF THE GENERA AND SPECIES OF MICRA8TERIAS 

 AND EUASTRUM. BY THE REV. R. V. DIXON, A. M., EX-F. T. C. D., CLOGH- 

 ERNEY RECTORY, DUNGANNON. 



I BEG leave to submit to the notice of your Society the following account 

 of a form of Desmid, which I have lately met with in this neighbour- 

 hood, and which I believe has not hitherto been described. The frond, 

 as represented in Plate I., Figs. 5-7, is simple, compressed, with a deep 

 and acute gaping constriction between its segments, which are three- 

 lobed, the line separating the extreme from the basal lobes being parallel 

 to the line of separation of the segments. It has no inflation on its surface, 

 but exhibits on its margin a few mucronate spines. This form appears 

 to me to be generically distinct from both Micrasterias and Euastrum : 

 from the former in the direction of the separation of its lobes, from the 

 latter in the absence of inflations. In these characteristics it agrees 

 with Micrasterias oscitans, and M. pinnatifida {Ralfs) as weU as'with the 

 form Holocystis oscitans (Hassall), described by Dr. Hassall, and referred 

 to by Mr. Ralfs ("British Desmidiae," pp. 69-77), and it is worth consi- 

 dering whether these forms should not be all grouped together in a new 

 genus. Before proceeding, however, to give the complete description 

 of this proposed genus and the three species which it would contain, I 

 beg leave to offer a few remarks on the different manner in which the 

 segments are divided in those Desmids which have lobed or divided 

 segments, and^which for this reason I take the liberty of calling Schizo- 

 merous Desmids. 



The typical mode of division (as exemplified in Euastrum pinnatuniy 

 E. oUongum, &c.) appears to be into three portions or subdivisions: 

 the first, next the line of separation of the segments, extending across 

 the frond, and embracing the two basal lobes ; the second, including the 

 median lobes ; and the third, the extreme or end lobe. This last, or 

 third subdivision, is the most constant. The two former are frequently 

 represented by a mere sinuosity or shallow indentation where the third 

 is distinctly developed, but we never find the first subdivision distinct, 

 and the second and third imperfectly separated. The whole three, in- 

 deed, maybe merely marked by slight sinuosities, as in Euastrum cune- 

 atum, but if any one is separated, it is the third, and this, I may observe, 

 is the order of development of the subdivisions in the growing segment 

 of the typical Micrasterias. The new segment is first hemispherical ; 

 the third subdivision is then developed; and afterwards the first and 

 second are separated. 



For the purposes of description these three subdivisions might be 

 denoted by the letters a, h, c, and their partial or complete development 

 marked as follows: — When the subdivisions are distinctly separated, 

 their symbols might be separated by commas, thus, a, h, c; when any 

 two or more are merely marked by a sinuosity, they might be repre- 

 sented thus, a^^h ; and if there is no trace of separation, thus, ab ; and 

 if, at the same time, the direction of the lines separating the subdivi- 



