88 1HMTESH DES.MlDIACE.i:. 



Genus 17. ARTHRODESMUS Elirenb. 1838. 



Ehrenb. in Arch. jSaturg. Bd. II, 1836, p. 185 [name only] ; Infus. 1838, 



p. 149. 



Ralfs in Ann. Mag-. Nat. Hist, xv, 1845, p. 150. 

 Hass. Brit. Freshw. Alg. 1845, p. 35(5. 

 Kalfs, Brit. Desm. 1848, p. 117. 

 Ktitz. Spec. Algar. 1849, p. 176. 

 Arch, in Pritch. Infus. 1861, pp. 721, 736. 

 Rabenh. Flora Enrop. Alg-. Ill, 1868, p. 225. 

 Kirchn. Alg. Schles. 1878, p. 156. 

 Wolle, Desm. U. S. 1884, p. 95. 

 Cooke, Brit. Desm. 1887, p. 134. 

 Hansg. Prodr. Algenfl. Bohin. 1888, p. 202. 

 De Toni, Syll. Alg. 1889, p. 1056. 

 Turn. Freshw. Alg-. E. India, 1893, p. 137. 

 G. S. West, Treatise Brit, Freslrw. Alg-. 1904, p. 170. 



Cells mostly small, sometimes minute, usually about 

 as broad as long, invariably compressed (except in the 

 very rare triangular forms), symmetrical in three planes 

 at right angles to each other; median constriction 

 generally fairly deep, si mix nearly <ilifuijs open-, semi- 

 cells very generally obverselv triangular, sometimes 



/ O ,' e/ O 



subquadrate and sometimes elliptic or subelliptic, 

 centre of semicell quite phnif and undifferentiated ; 

 vertical view elliptic, icitli i> median protuberances. 

 Cell-wall furnished wit/t, simple spin ex disposed in a 

 median plane, one inserted at each lateral angle. 

 Chloroplasts axile, generally one in each semicell, 

 but subject to considerable variation, usually with 

 one central pyrenoid. 



Zygospores globose, smooth or furnished with simple, 

 subulate spines. 



The genus Arthrodesmus is closely related to "Kanthidium 

 and to certain of the spiny species of Staurastrum. It is 

 primarily distinguished from Xanthidium by the absence 

 from the semicells of any differentiated central area, and 

 secondarily by the insertion of the spines in a median plane. 

 Paired spines do not occur in the genus Arthrodesmus except 

 in the one instance of A. tenuissimus Arch., in which 

 the minute apical spines are paired. On the other hand, 

 there are several species of Xanthidium in which all the 

 spines are median and unpaired. 



Some forms of certain species of Staurastrum, sucli as St. 



