COSMARIOI. 183 



the middle of eacli semicell there is a large central protuber- 

 ance with concentrically arranged granules, and the o-eiieral 



t/ O o 



granulation of the lobes becomes slightly reduced towards 

 this central tumour. The apex of the median lobe is slightly 

 retuse, but there is no reduction of the granulation at this 

 part. Nageli's original figures of this Desmid are most 

 inaccurate,, especially as regards the granulation. Although 

 that author clearly shows the presence of the central pro- 

 tuberance in his figure of the vertical view,, he does not show 

 the slightest indication of it in his figure of the empty semi- 

 cell. We have examined many British and North American 

 specimens of this species, and have never yet seen one with- 

 out the central protuberance with its concentrically arranged 

 granules. 



There is no doubt whatever that C. ornatum v&r. protractum 

 Wolle, C. ornntuui var. minor Wolle, and C. ornatum var. 

 poloniciim Racib. are forms of C. protractum (Nag.) De Bary. 

 very indifferently described and figured, insufficient attention 

 having been given to accuracy in depicting* the granulation. 



C. protractum exhibits certain variations in the apical lobe 

 of the semicells. This lobe is sometimes relatively short and 

 slightly constricted at the base,, whereas in its more usual 

 condition it is very gradually attenuated upwards. These 

 modifications can be observed in different individuals in the 

 same collection. 



174. Cosmarium Corbula Breb. 

 (PI. LXXXII, figs. 9-11.) 



Cosmarium Corbula Breb. Liste Desm. 1856, p. 131, t. 1, f . 13 ; Arch, in 

 Pritch. Infns. 1861, p. 734 ; Cooke, Brit. Desm. 1887, p. 107, t. 43, f. 9 ; 

 De Toni, Syll. Alg. 1889, p. 1054 ; Koy & Biss. Scott. Desm. 1894, p. 44, 

 t. 2, f. 18; Nordst. Index Desm. 1896, p. 82 ; West & G. S. West, Alg. 

 S. England, 1897, p. 490 ; Alga-fl. Yorks. 1900, p. 76 ; Alg. X. Ireland, 

 1902, p. 39 ; Freshw. Alg. Orkneys and Shetlands, 1905, p. 20. 



C. Sportella Breb. b. Corbula Eabenli. Flor. Europ. Alg. Ill, 1868, p. 169. 



Ursinella Corbula Knntze, Revis. gen. plant. 1891, p. 924. 



Cells rather small, about as broad as long (or very 

 slightly broader), deeply constricted, sinus very nar- 

 rowly linear; semicells broadly pyramid ate-trapeziform, 

 basal angles rounded, upper parts of sides slightly 

 retuse just under the apex, apex broadly truncate and 

 straight, apical angles very slightly emarginate and 

 bigranulate. Cell-wall granulate, granules restricted 



