THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



SUPPLEMENT TO VOL. XIV. JANUARY 1845. 



LV. — On the British Desmidieae. By John Ralfs, Esq., 

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



[With a Plate.] 



Xanthidium, Ehr. 



Fronds simple, constricted in the middle ; segments slightly com- 

 pressed, turgid and entire, having their surfaces more or less 

 furnished with simple or branched spines. 



In this genus the fronds are deeply constricted in the middle ; 

 their segments are slightly compressed, turgid, reniform or or- 

 bicular, and entire, rarely somewhat angular, and they are fur- 

 nished with elongated spines which are simple or branched, and 

 either scattered over the surface or confined to the margin, when 

 they are apparently placed in two rows, one on each side of the 

 marginal line. 



Xanthidium requires to be distinguished from two genera only, 

 namely Staurastrum and Cosmarium; its resemblance to the 

 former is more apparent than real, and will probably mislead no 

 one who has examined a plant belonging to that genus. In Stau- 

 rastrum the processes are elongations of the angles, and in an 

 end view always marginal ; whereas in Xanthidium the spines are 

 situated on the convex surface, are arranged in transverse series, 

 and consequently in an end view run in lines across the centre. 

 The connexion with Cosmarium is far more intimate, and in fact 

 the sole distinctive character that can be relied upon is the pre- 

 sence of elongated spines in this genus. Meneghini indeed still 

 more reduces the number and value of the differential marks by 

 retaining in Xanthidium only those species whose spines are scat- 

 tered over the surface, whilst he refers to Cosmarium these in 

 which the spines are confined to the margin ; I think however 

 that his views cannot be admitted, and that we must either retain 

 the genus as formed by Ehrenberg or unite all the species with 

 Cosmarium, for in Xanthidium furcatum the principal spines are 

 marginal, and many specimens, especially in a young state, have 

 none other. 



In two species a remarkable projection on the centre of each 



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

 Ann. ^ Mag. N. Hist. Vol. xiv. SuppJ. 2 I 



