COSMARIUM. 125 



Genus 15. Cosmarium Cor da, 1834. 



Corda in Aim. de Carlsbad, 1834. 



Ealfs in Ann. Mag. Nat. Hist. 1844, p. 391. 



Hass. Brit, Freshw. Alg. 1845, p. 361. 



Ealfs, Brit. Desin. 1848, p. 91. 



De Bary, Conj. 1858, p. 72. 



Arch, in Pritch. Infns. 1861, pp. 720, 731. 



Eabenh. Flor. Europ. Alg. 1868, III, p. 157. 



Delp. Desm. subalp. 1873, p. 38. 



Kirchn. Alg. Schles. 1878, p. 145. 



Wolle, Desm. U.S. 1884, p. 57. 



Cooke, Brit. Desm. 1886, p. 79. 



Hansg. Prodr. Algenfl. Bolim. 1888, p. 192. 



De Toni, Syll. Alg. 1889, p. 931. 



Wille in Naturl. Pflanzenfam. 1899, p. 10. 



a. S. West, Treatise Brit. Freshw. Alg. 1904, p. 166. 



Cells very variable in size, usually a little longer 

 than broad (sometimes 1-J- times or rarely about twice 

 as long as broad), generally somewhat compressed, 

 symmetrical in three planes at right angles to each 

 other ; constricted in the middle, often deeply, but 

 sometimes very slightly; semicells very variable in 

 outline, elliptical, subcircular, semicircular, subquad- 

 rate, or truncate-pyramidate, apex rounded, truncate 

 or subtruncate, without an apical incision, centre of 

 the senricell sometimes protuberant; vertical view ellip- 

 tic, subelliptic, or oblong (rarely circular), often with a 

 protuberance (sometimes more than one) at the middle 

 on each side. Chloroplasts usually axile, one in each 

 semicell, with four curved longitudinal plates and a 

 large central pyrenoid, or with two large pyrenoids 

 transversely disposed ; in a few species parietal, 4-8 

 in each semicell, with one or more pyrenoids in each 

 chloroplast. Cell-wall smooth, punctate, scrobiculate, 

 granulate, verrucose, or papillate, the various markings 

 usually being arranged in a definite pattern. 



Zygospores very variable, globose, ellipsoid, or quad- 

 rate in form, smooth, scrobiculate, papillate, or furnished 

 with furcate, or more rarely simple, spines. 



The genus Cosmarium is the largest of the genera of 

 Desmids, and includes several hundred species. Ooe of the 



