Mr. J. Ralfs 07i the British Desniidicfe. 391 



Vesicle of Thoa miiricaia. Fig. 7. Idea of the structure of this form of ve- 

 sicle, whicli is derived from the shortening of the axis and the whorling and 

 transformation of the cells. 



III. Vesicles of the third order. Fig. 8. Vesicle and branch of Sertiilaria 

 rosacea. Fig. D. ^'esicle of a Sertularia, in which an internal framework 

 and axis are seen after the exclusion of the ova. 



IV. Vesicles of the fourth order. Fig. 10. A Sertularia, in which the se- 

 condary branches are grouped in pairs ; in this specimen two of the branches 

 are converted into vesicles, but retain their original position and relation to 

 their twin branches. Fig. 1 1. Vesicle of a Sertularia belonging to this order, 

 exhibiting longitudinal dehiscence. 



V. Vesicles of the fifth order. Fig. 12, Vesicle, and fg. 18, part of the 

 branch of Thoa Beanii. 



VI. Vesicle of the sixth order. Fig. 14. Vesicle and polypc-ccll of a 

 Laomedea. 



XLVIIL— On the British Desmidiefe. By John Ralfs, Esq., 

 M.R.C.S., Penzance*. 



[With a Plate.] 



CosMARiUM, Corda. 



Fronds simple, constricted in the middle ; segments as broad as 



or broader than long, neither sinnated nor notched. 



The fronds are very minute, simple, constricted in the middle ; 

 the segments are generally broader than long and inflato-com- 

 pressed, are neither emarginate at the end nor lobed at the sides, 

 and have no spines or processes. One species only is longitudi- 

 nally inflated down the middle, and therefore appears lobed in 

 the end view. In the rest this view is most frequently elliptic ; 

 in some species however it is circular. 



Ehrenberg united plants belonging to this genus with others 

 having loljed segments in order to form his genus Eiiastriim. 

 T\Ieneghini for the most part followed this arrangement, merely 

 changing the name to Cosmarium, as having the prior claim, and 

 also adding some species taken from Xanthidium. Under Mi- 

 crasterias, Euastrum, and Xanthidium I have given my reasons 

 for differing from such high authorities, and also pointed out the 

 characters which distinguish those genera from Cosmarium. 



Both Ehrenberg and Meneghini consider the inflato-com- 

 pressed segments essential, but some species (presently to be de- 

 scribed) with globular or cyhndrical segments can by no means 

 be separated from the compressed ones. Those having cylindrical 

 fronds in some respects show an affinity with Closterium, and 

 perhaps Closterium Cylindiiis would be more suitably united 

 with them. In Cosmarium the fronds are never elongated nor 

 curved, and are always constricted in the middle, and the repro- 

 ductive granules are scattered. 



- * Read before the Botanical Society of Edinburgh, June 13th, 1844. 



