ARCHER AND DIXON, ON DESMIDIACEvE. 81 



marked by their peculiar inflations, a few of which are 

 parallel, but the majority subradial, next; and the Micras- 

 terias last, terminating with the radial closed species, from 

 which the transition would be easy to the Cosmaria, among 

 which traces of radiation still appear in the endochrome of 

 C. Ralfsii, in the ridged surface 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 M. oscitans, M. pinnatifida, 

 Ralfs, Holocystis oscitans, Hassall — if this be distinct from 

 M. oscitans, which appears doubtful — and our new form ; the 

 next being Euastrum, and the last Micrasterias. In conclu- 

 sion, I beg to mention that I owe the drawing which accom- 

 panies this paper to the kindness of Mr. Archer, whom I 

 consulted when I first met with the new form under discus- 

 sion, and to whom I forwarded the gathering in which it 

 occurred for further examination; and that the following 

 detailed generic and specific descriptions have been drawn up 

 by the same gentleman. 



Family Desmidiace,e. 

 Tetrachastrum {gen. nov.)* 



Generic characters : Frond simple, compressed, deeply 

 divided into two three-lobed segments ; the basal lobes pro- 

 jecting horizontally, broadest within and attenuated outwards ; 

 end lobe expanded into two lateral attenuated projections 

 parallel in their direction with the basal lobes ; ends straight, 

 or convex, or having at the middle of the rounded ends a 

 very slight concavity. 



General generic description : The fronds are simple, as 

 long as or longer than broad, compressed, without inflations, 

 deeply divided into two segments by a constriction, forming a 

 broad acute-angled notch; each segment constricted by a 

 broad notch or sinuosity upon each side into two subdivisions 

 forming three lobes, the basal lobes broadest within and 

 attenuated outwards, not radial, but extending horizontally 



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

 the fronds, which is most conspicuous in T. oscitans and T. pinnatifidum, and 

 aarpov, a star. This latter 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 1 

 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. cuneatum, &c, for instance. — 

 R. V. D. 



