XANTHIDIUM. 49 



Cells of very variable size, usuallv some \vliat longer 



i/ O 



than broad, invariably compressed (except in the rare 

 triangular forms), symmetrical in three planes at 

 right angles to each other; median constriction invari- 

 ably deep; semicells of variable outline, elliptical, 

 elliptic-hexagonal, trapeziform, or polygonal, generally 

 with a flattened apex, centre <>f flic semicell (with rare 

 exceptions) differentiated as a thickened, often scrobicu- 

 lated, and generally protuberant area; vertical view 

 more or less elliptic, generally with a protuberance at the 

 middle on each side. Cell-wall furnished with simple, 

 or more rarely u'ith furcate spines, sometimes disposed 

 in a median plane, but -usually ar 'ran aed more or less 

 symmetrically on each side of a median plane. Chloro- 

 plasts generally parietal, arranged in four cushions in 



JT o / 1 o 



each semicell, each with one or more pyrenoids, but 

 often irregular, partly parietal and partly axile ; 

 chloroplasts axile in many of small species, with a 

 single central pyrenoid in each semicell. 



Zygospores globose, usually furnished with simple 

 or furcate spines of variable length, more rarely spine- 

 less and conspicuously scrobiculate. 



The genus Xanthidium is distinguished by the compressed 

 cells, the symmetrical, and generally paired, arrangement of 

 spines, and by the differentiated central area of the semicells. 

 The last character is one of the most important features of 

 the genus, although it is often very slight, and is entirely 

 absent in a few forms (such as X. antilop&um var. l&ve, X. 

 controversum, and X. crixtatu m var. leiodermum) . The differen- 

 tiated region may take the form of a thickened area, with or 

 without scrobiculations, or of a protuberance of variable size, 

 which may be entire, granulate, dentate, or spinate. 



The genus is nearly related to Cosmarium, Arthrodesmus, 

 and Staurastrum, from the first of which the majority of the 

 species may have been evolved. It is mostly in the tropics 

 that transitional species occur which connect the genera 

 Cosmarium and Xanthidium. Turner remarks about the 

 Indian species Xanthidium cosmariforme that " this might 

 just as correctly be called Cosmarium xanthidiforme." 



Lundell, in 1871, instituted the two subgenera Schizacan- 

 thnm and Holacanthum, the former to include the species 



VOL. IV. 4 



