80 ARCHER AND DIXON, ON DESMIDIACE^. 



which for tliis reason I take the liberty of calling Schizomerous 

 Desmids. 



The typical mode of division (as exemplified in Euasti-um 

 pinnatum, E. oblongum, &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 sub- 

 division, 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, indeed, may be merely marked 

 by slight sinuosities, as in Euastrum cuneatum, 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 seg- 

 ment 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, b, c ; and their partial or 

 complete development marked as follows : — When the subdi- 

 visions are distinctly separated, their symbols might be 

 separated by commas, thus, a, b, c ; when any two or more 

 are merely marked by a sinuosity, they might be represented 

 thus, a^~~b ; and if there is no trace of separation, thus, ab ; 

 and if, at the same time, the direction of the lines separating 

 the subdivisions were noted, the full description as regards 

 the divisions of the segments would be given. Thus : 



Euastrum cuneatum would be represented by ci~^b'~^c. 



„ pinnatum „ „ a, b, c, parallel. 



„ oblongum „ „ a, b, c, subraidal 



Micrasterias deuliculata „ „ a, h, c, radial 



Euastrum pectinatum „ „ tf~*6, c, parallel. 



And our new form „ „ ab, c, parallel. 



The direction of the lines of separation of the subdivisions in 

 the Schizomerous Desmids varies from parallelism to true 

 radiation, and at the same time the intervals between the 

 subdivisions close, so that in Micrasterias denticulata, rotata, 

 and fimbriata, the frond appears almost entire with radial 

 lines on its surface. I think regard ought to be had to this 

 characteristic in placing the genera and Bpeciea of Euastrum 

 and Micrasterias between the filamentous forms on the one 

 hand and the Cosmaria on the other; that the forms with 

 parallel subdivisions should come first; the Euastra, so well 



