DVBIIW NATTTHAl HISTORY 80CIETT. 208 



sions were noted, the ftill description as regards the divisions of the seg- 

 ments would be given. Thus — 



Eua»trwn cuneatum would be represented by oT^h^^o. 

 Euastrum pinnatum, „ „ «, J, <?, parallel. 



Euaatrum ohlonguMy „ „ a^ h, Cy subradial. 



Micrasterias denticulatay ,, <», i» Cj radial. 



Euastrum pectinatuniy ,, ,, a^^J, c, parallel. 



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



The direction of the lines of separation of the subdivisions in the Schizo- 

 merous Desmids varies from parallelism to true radiation, and at the 

 same time the intervals between the subdivisions close, so that in 

 Micraaterias denticulata, rotata, and Jimhriata, 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 species 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 marked by their peculiar inflations, a 

 few of which are parallel, but the majority subradial, next ; and the 

 Micrasterias last, terminating with the radial closed species, from which 

 the transition would be easy to the Cosmaria, among which traces of 

 radiation still appear fti the endochrome of C. JRalfsti, in the ridged sur- 

 face of C. undulatum, and the crenated margin of other species. The 

 whole group of Schizomerous Desmids then might be distributed among 

 three genera — the first containing Jf. oscitans, M. pinnatifida (Ralfs)^ 

 Holocystia oscitans {Hassall) — if this be distinct from M. oscitamy which 

 appears doubtful — and our new form ; the next being Euastrum, and 

 the last Micrasterias. In conclusion, I beg to mention that I owe the 

 drawing which accompanies this paper to the kindness of Mr. Archer, 

 whom I consulted when I first met with the new form under discussion, 

 and to whom I forwarded the gathering in which it occurred for farther 

 examination ; and that the following detailed generic and specific de- 

 criptions have been drawn up by the same gentleman. 



Family.— DESMTDIACE-^. 

 Tetrachasteijm {gen. nov.).* 



Generic Characters : Frond simple, compressed, deeply divided 

 into two three-lobed segments ; the basal lobes projecting horizontally, 

 broadest within and attenuated outwards ; end-lobe expanded into two 

 lateral attenuated projections parallel in their direction with the basal 



* From Ttrpaxa, in four parts, in reference to the fourfold division of the fronds, 

 which is most conspicuous in T. oscitans and T. pinnatijidumy and aorpovy a star. This 

 Utter term, in its usual sense of a radiate form, is not a descriptive one, as applied to our 

 new genus ; but I adopt it because it occurs in the names of the other two genera of the 

 same group, and I wish to mark their mutual affinity. Moreover, the term avrpov is 

 not more inapplicable, on this ground, to the fronds of the proposed genus than it is to 

 those of several species of Euastra, — E. euneatvmy &c., for instance. — R V. D. 



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