358 DESMIDE^I. 



quadrangular, each angle produced into a spine. End 

 view subelliptic, with a spine at each extremity. 



Arthrodesmus octocornis Ehr., Infus. p. 152. S.? octocorne 



Ralfs, in Annals, vol. xv. p. 159. pi. xii. fig. 3. 

 Hob. Boggy ponds near Dolgelly, rare : Mr. Ralfs. 



" The newly formed segments at first have only two 

 spines, and in this state somewhat resemble those of Staur- 

 astrum Incus, of which indeed this plant may eventually 

 prove a variety. But 8. Incus has only two spines on each 

 segment, and its end is not concave but truncate." Ralfs. 



The Micrasterias octocornis of Meneghini would appear to 

 be a different plant. 



56. XANTHIDIUM Ehr. 



Char. Frond composed of two slightly compressed segments 

 constricted in the centre, neither lobed, sinuated, nor emar- 

 ginate. Surface clothed with simple or branched elon- 

 gated spines, either scattered or confined to the margin, 

 where they are apparently placed in two rows, one on each 

 side the marginal lines. Segments anteriorly perforated 

 in the centre (?). 



Derivation. From %av6o$, yellow ? 



This genus bears a close resemblance to Cosmarium, from 

 which there is no satisfactory character to distinguish it, 

 unless indeed the projection or aperture noticed to occur by 

 Mr. Dalrymple in Xanthidium furcatum, and by Mr. Ralfs 

 in X. aculeatum be constant in all the true species of the 

 genus Xanthidium. Meneghini even goes so far in the union 

 of the genera Xanthidium and Cosmarium, as to retain in the 

 former genus only those species whose spines are scattered 

 over the surface, while he refers to the latter genus those in 

 which the spines are confined to the margin. This arrange- 

 ment seems arbitrary ; and that it is so, is proved by the fact 

 that in a species of Xanthidium figured by Dr. Bailey in the 

 " American Bacillaria/' and which has six pairs of marginal 



