IVi'si : Phvtoplankton of English Lake District. 325 



25. DiNOBRYON CREXULATUM Sp. 11. (Fig. 7C and d). 

 Membrana evacuata firma et hyalina, elongato-campanulata, 

 parte basali leviter dilatata et in stipitem tenuem brevem 

 producta, lateribus leviter concavis ; margine toto undnlato- 

 crenulato, crenulis 9-12 utrobique. 



Long. cum. stip. 31-32 /><.; long. stip. 2 /x; lat. bas., 9.5-10 fi; 

 lat. med., 7.5-8 /x ; lat. oscul., 8-8.5^1. 



Kah. In the plankton of Ennerdale Water (May 1903). 



Only solitary individuals of this species were seen, and we 

 have no evidence to show that colonies are ever formed. It 

 differs from all the known species in its completely crenulate 

 wall from base to apex. 



Peridinie.e. 



26. Ceratium hirundinella O. F. Miiller. This species 

 is the most generally distributed of all the Peridinieae in the 

 British freshwater plankton. There is a strange absence from 

 the English lakes of the common four-horned form which is so 

 plentiful in the Scottish and Irish lakes (consult ' Trans. Roy. 

 Soc. Edin.', XLL, 1905, part III., p. 494, fig. i c and d ; and 

 ' Trans. Roy. Irish Acad.', XXXIII. , sect. B. 1906, part II., 

 p. 94, figs. 6-8). The only four-horned form observed occurred 

 in Windermere, and all the horns were stunted, especially the 

 apical horn (Fig. 8 C). A form similar to this has been observed 

 by Bachmann in the Thunersee, and also in the Zugersee, in 

 Switzerland. x\ll the other forms seen were three-horned, ex- 

 cept those from Ulleswater and Hawes Water, which possessed 

 a trace of the fourth horn (fig. 8 e). The commonest forms 

 observed were three-horned, with the median antapical horn 

 disposed parallel to the longitudinal axis. 



In no single instance was Ceratium hirundinella observed 

 abundantly in the English lakes. It is general and frequent, 

 but does not appear to form large maxima. 



27. Ceratium cornutum (Ehrenb.) Clap. & Lachm. This 

 species is much less frequent than the preceding, and was only 

 observed from Brothers' Water and Grasmere (fig. 8 F-H). 

 There is much variability in the curvature of the horns, and 

 we are inclined to believe that C. curvirostre Huitfeldt-Kaas is 

 only a form of it. 



28. Peridinium Willei Huitfeldt-Kass in ' Vidensk. 

 Skrifter,' 1900, No. 2, p. 5, fig. 6-9 ; Borg. & Ostenf. in 

 ' Botany of the Faeroes,' 1903, p. 622, fig. 150 ; Lemm. in 

 ' Archiv fur Hydrobiol. u. Planktonkunde/ III., 1908, p. 376, 



1409 Sep. I. 



