XANTHIDIUM. 71 



Schles. 1878, p. 155 ; Wolle, Desui. U. S. 1884, p. 93, t. 21, f. 6, and 7 ? ; 

 Cooke, Brit. Desm. 1887, p. 133, t. 46, f. 3 ; Boldt, Desmid. Grimland, 

 1888, p. 31 ; De Toni, Syll. Alg. 1889, p. 923 ; Heinierl, Desm. alp. 

 LS9L, p. 595 ; Hans-?. Prodr. Algenfl. Bohm. 1892, p. 252 ; West, Alg. W. 

 Ireland, 1892, p. 165 ; Alg. Engl. Lake Distr. 1892, p. 730 ; Eoy & Biss. 

 Scott. Desm 1893, p. 244; Nordst. Index Desmid. 1896, p. 89; W. & G. 

 S. West, Alg. S. England, 1897, p. 484 ; Alga-fl. Yorks. 1900, p. 67 ; Scott. 

 Freshw. Plankton, I. 1903, p. 527 ; Larsen, Freshw. Alg. E. Greenland, 

 1904, p. 101 ; W. & G. S. AVest, Further Contrib. Freshw. Plankton 

 Scott. Lochs, 1905, p. 485 ; Comp. Study Plankton Irish Lakes, 1906, 

 p. 85 ; Borge, Beitrage Alg. Schweden, 1903, p. 28 ; Larsen, Ferskvand- 

 salg. Vest-Gronl. 1907, p. 354 ; W. & G. S. West, Brit. Freshw. Phyto- 

 / plankton, etc. 1909, p. 180. 



Didymiditim (Xanthidium) cristatum (Breb.) Reinsch, Algenfi. Frank. 1867, 

 p. 127. 



Xanthidium cristatum var. retiiforme Cooke, Brit. Desm. 1887, 133. 



Holocanthum cristatum (Breb.) Wille in Eng'ler & Prantl, Natiirl. Pflan- 

 zeiifam. 1890, pp. 11, 12 (c. fig-. 7 D). 



Cells of medium size, a little longer than broad 



* O 



(without spines), deeply constricted, sinus rather 

 variable, sometimes narrowly linear with a dilated ex- 

 tremity and sometimes slightly open; semicells 

 trapeziform-subsemicircular, with a broad, subreni- 

 form, flat, or somewhat convex base, and with a basal, 

 lateral and apical angle on each side, apex truncate 

 and generally straight, basal angles furnished with a 

 single slightly convergent spine, lateral and apical 

 angles each furnished with a pair of divergent spines, 

 spines all simple and straight, central area differen- 

 tiated into a small thickened protuberance, generally 

 with a slight vertical elongation. Side view of semi- 

 cell circular, with a more or less flattened protuberance 

 at the middle 011 each side, and with a pair of divergent 

 spines at the apical margin. Vertical view elliptic, 

 with a small protuberance at the middle on each side, 

 and three slightly divergent spines at each pole. Cell- 

 wall delicately punctate. 



Zygospore globose, furnished with numerous, elon- 

 gated, stout spines which are very slightly emarginate 

 at the apex. 



Length without spines 44-55 /*, with spines 60-77 //, ; 

 breadth without spines 84' 5-48 p., with spines 48-68 ^ ; 

 breadth of isthmus 13-15'5/x; thickness 25-32*5 ^ ; 

 diam. zygosp. without spines 48*5-51 //,, length of 

 spines 15-16 //. 



