FLETTROT 1'XIUM. 209 



This is ono of tlie rurest British species of tin- ^enus, and is 

 well characterised by its sni;ill si/e and apical tcctli. Tlie 

 four apical troth ;ire not all in focus at Hie same time, and 

 1 ln's led to an error in Wolle's original description, in which 

 lie mentions only three teeth. It is a decided western species 

 in the British Isles. .Some forms were noticed which were 

 suddenly attenuated at about one fifth of the length of their 

 semicells from the apices. 



As this DesmitT is not " ( '/<>xt<'ri,u,'i>i. Sceptrum Kiitz. 1845," 

 that species being- Dofitlhi in Jj/tr/i/u m [>rel>., IJoy's specific 

 name " i^ci-fif m in " cannot be accepted for the species, bring 

 one year subsequent to Wolle's Dor/'dhini tridentulum. 



Var. capitatum West. (PL XXX, figs. 7, 8.) 



Pleurotsenium tridentulum var. capitatum West, Alg. W. Ireland, 1892, 



p. 120, t. 24, f. 12. 



/'/. triiloit itliim vav. granulatum West, 1. c. p. 120. 



PI. Sceptrum West A: G. S. West, var. cn.pitatnm West <t G. S. West, Sonic 

 N. Amer. Desm. 1896, p. 235, t. 13, f. 7, 8. 



Cells more elongate than in tlie type, 26-27 times 

 longer than their diameter; semicells gradually 

 attenuated to the apices, which are subcapitate 

 and rounded ; cell- wall granulate except at the 

 apices, which are smooth and furnished with the 

 usual four teeth. 



Length 314-360 /z ; breadth at base of semicells 

 12-14 /a; breadth of apices 7'5-8'5 /JL. 



ICELAND. Lakes, Clifden to Roundstone, Galway ! 



Geot/r. Distribution. United States. 



6. Pleurotsenium Trabecula (Ehrenb.) Nag. 

 (PL XXX, figs. 11-13.) 



Closterium Trabrniln Ehrenb. Beitr. zur Kenntniss dor Organis. dcr 

 Infus. 1830, p. G2 and 70 ; Ehrenb. Infns. 1838, p. 92, t. H, f. II (in pail : 

 according- to Turner only 114 and 117) ; Monegh. Synops. Desm. 18JO, 

 p. 235. 



Plevrntii'iihnii Trabecula (Ehrenb.) Nag. Gatt. einz. Alg-. 1849, p. 104, 

 t. 6, f. A ; ? Eabenh. Flor. Enrop. Algar III, 1808, p. 141 ; Laind. Desm. 

 Sncc, 1871, p. 89 ; Nordst. Alg. aq. dulc. et Char. Sandvic, 1878, p. 11 ; 

 ' J Kirchn. Alg. Schles. 1878, p^ 144; Hansg. Prodr. Algenfl. Bohnr. 18ss, 

 p. 190 ; De Toni, Syll. Alg. 1889, p. S95 ; Borge, Chlor. Norska Finmark. 

 1892, p. 13; Nordst. Index Desm. 1896, p. 255; West & G. S. West, 

 Desm. Singapore, 1897, p. 159 ; Alg. S. England, 1897, p. 483 ; G. S. W.-st . 



14 



