VANHEURCKIA. 



239 



C. lacustre (C. Agardh.) H. Van Heurck. (Schizontma 



lacuslre C. Agardh! Colletonema subcohizrens Thwaites. H.V.H. Atl., pi. 15, 

 f. 40* ; Type No. 155), plate 5, fig. 246. 



Valves lanceolate, gently attenuate, with apices obtuse, sometimes very 

 slightly rostrate, one of the sides of the valve often narrower than the other. 

 Striae robust, finely divided transversely, very radiate at the median portion of 

 the valve, then convergent and becoming again very radiate at the apices, 9 in 

 1 cd.m.j more distant and shorter or absent round the central nodule. 

 Length, 3 to 6 c.d.m. 



Fresh water. England (W. Sm.), Sweden. Not yet found in Belgium. 



GENUS 11. 



BREB., 1868, 





VANHEURCKIA 

 Char. Emend. 



Frustules naviculoid, free, or very rarely enclosed in single file in 

 a membranous tube. Valves with striae fine, parallel, rarely slightly 

 radiate at the median portion of the valve. Central and 

 terminal nodules linear-elongated between the two branches 

 of a double raphe. 



Prof. Cleve has re-established the genus Frustulia of Agardh 

 to receive Vanheurckia. This proposal appears untenable. 

 Agardh defined his Frustulia (Syst. Alg., 1824, p. 13) as " Frustula 

 acicularia linearia vel cuneata muco obvoluta in corpus lineare 

 aggregata," and in his Conspectus Crit. Diatomacearum (1830, 

 p. 43) he repeats, " Frustula linearia, libera vel muco amorpho 

 immessa, solitaria vel binatim conjuncta." These definitions say 

 practically nothing at all, and accordingly both he and his 

 successors have accumulated in this genus anomalous forms : 

 F 'g- 39- Amphora, Navia/la, Synedra, etc. The genus established by de 



Vanheurckia . . . . 



vulgaris. Brebisson on the other hand has very distinct characteristics, and 

 deserves to be preserved, and more justly so than many others that have 

 been admitted in this work. 



The genus includes 5 or 6 species, of which the most beautiful is V. 

 Lewisiana Breb., which is found in America, India, and Guiana. 



m 



Strine quite parallel, raphe 

 continued over all the valve. 



ANALYSIS OF SPECIES. 



Valve rhomboidal-lanceolate ; endo- 

 chrome yellowish. 



. V. rhomboides. 



Valve lanceolate-elongate ; endochrome 



greenish. . . .V. viridula. 



k Median striae slightly radiate ; 1 aphe interrupted near the central nodule V. vulgaris. 



