BRITISH DESMIDIE^. Ill 



Length of frond -^ of an inch ; greatest breadth -^ ; breadth at con- 

 striction -gT^. 



Tab. XXXII. fig. 8. a. mature frond ; b. empty frond ; c. transverse \iew. 



9. XANTHIDIUM, Ehr. 



Frond simple, constricted in the middle ; segments compressed, entire, 

 spinous, having a circular projection near the centre, which is 

 usually tuberculated. 



In this genus the frond is simple and deeply constricted at the 

 middle ; its segments are slightly compressed, turgid, entire, either 

 reniform, orbicular or angular ; they are furnished with spines, which, 

 simple or branched, are either scattered or confined to the margin ; 

 in the latter case they are arranged in two rows, one on each side of 

 the marginal line. Near the centre of each segment is a remarkable 

 circular projection on both surfaces. These projections are usually 

 surrounded by a beaded circle of pearly granules or tubercles, which 

 are most evident in a lateral or transverse view. 



Xanthidium requires to be distinguished from three genera, viz. 

 Siaurastrum, Arthrodesmus, and Cosmarium. Its resemblance to Stau- 

 rastrum is more apparent than real, and wiU probably mislead no one 

 who has examined a plant belonging to that genus, in which the cells 

 are angular irrespective of the processes. In Arthrodesmus each seg- 

 ment has only two spines, one on each side, and there are no central 

 projections. 



The connection with Cosmarium is far more intimate ; In fact the 

 sole distinctive character that can be rehed upon is the presence of 

 spines in this genus. Meneghini indeed still further reduces the 

 number and value of the differential marks, by retaining in Xanthi- 

 dium merely those species whose spines are scattered over the surface, 

 whilst he refers to Cosmarium those 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 X armatum the 

 principal spines are marginal, and many specimens, especially in a 

 young state, have none other. 



I have followed preceding writers in taking the specific characters 

 chiefly from the form and position of the spines ; but Mr. Jenner con- 



