276 



ACHNANTHIDIUM. 



var. marinum. (G. marinum JV. Stn. ; H.V.H. Atl , pi. 26, f. 4; 

 Type No. 225), plate 7, fig. 320. 



Differs from the preceding by its size being generally larger, and its 

 marine habitat. 



R. Van Heurckii Grun. (H.V.H. Atl., pi. 26, .1. 5, 6, 7, 8, and 

 9*), plate 7, fig. 321. 



Very small. Valves broadly lanceolate-subcuneate. Upper valve with 

 striae radiate, submarginal, 14 or 15 in 1 c.d.m. Lower valve with striae reach- 

 ing nearly to the raphe, about 18 in 1 c.d.m. Length, rather less than 1 

 c.d.m. (7 to "9 c.d.m.). 



Fresh water. Very rare? Brussels, Botanical Gardens (Del.). 



This curious species has been found at Auvergne (Heribaud), and will 

 probably be met with elsewhere. 



Tribe IV. Achnanthe.e. 



: 



Valve with straight raphe 

 Valve with sigmoid raphe 



. Admanthes. 

 . Achnantliidium. 



GENUS 29. ACHNANTHIDIUM (KUTZ ) GRUN., 1880. 



Valves elliptic, strongly inflated at the median portion. Raphe 

 sigmoid. Upper valve with only a pseudo-raphe ; lower valve with a 



a. 



b. 



Fig. 60. Achnanthidium fiexellum. 

 a. Upper valve. (<. Lower valve, c , Frustule. 



