294 Bacillariece 



freshwater species ; the second subgenus Stauroneis I prefer to 

 regard as a genus ; and in the third subgenus there are three 

 sections which are exclusively marine. 



There are about 70 freshwater species occurring in the British Islands, 

 principally in stagnant waters. The most striking are N. cuspidata Kiitz., 

 N. nobilis Ehrenb., N. major Kiitz., N, alpina Ralfs (fig. 136 A) and N. lata 

 Breb., the two last-mentioned species preferring boggy tracts in elevated 

 regions, in which localities they are sometimes frequent. N. perpusilla Grun. 

 (length 12'5 p) and N. gallica (W. Srn.) V. H. (length 8 15 p.) are amongst 

 the smallest British species ; N. nobilis Ehrenb. is the largest species (length 

 200 400 p.} and N. viridis Kiitz. (fig. 136 B) is perhaps the commonest. 

 Some of the small and abundant species, such as N. exilis Grun. and others, 

 are remarkable for the rapidity of their movements. Some of the species of 

 this genus are utilized as test-objects for the objectives of microscopes. 



Genus Stauroneis Ehrenb., 1843. [Pleurostauron Rabenh., 

 1859 ; Schizostauron Grun., 1867.] This genus is at once dis- 

 tinguished from Navicula by the form of the central nodule, which 

 is transversely dilated to form a stauros. The frustules may be 

 solitary and free-floating or attached by their valve-faces to form 

 short filaments. 



There are about six freshwater species occurring in Britain, of which St. 

 Plicenicenteron Ehrenb. is the largest and most frequent; length of valves 

 100 170 /*; fig. 136 E. 



Genus Vanheurckia Bre"b., 1868. [Frustulia as amended by 

 Rabenh., 1851, but scarcely the Frustulia of Agardh, 1824.] The 

 frustules, which are free-floating or occasionally arranged in a 

 linear series in a mucous tube, are of precisely the same type 

 as those of Navicula. They possess, however, a very distinctive 

 feature in the sagittal line of the valves. There are distinct polar 

 and central nodules, which are elongated and enclosed along with 

 the raphe between two parallel siliceous ribs. The valves are 

 transversely striated, the striae being very fine and parallel, rarely 

 slightly radiating in the median portion of the valve. 



There are only two British species, V. rhomboides (Ehrenb.) Breb. 

 [= Navicula rhomboides Ehrenb.; fig. 137 A and B] and V. vulgaris (Thw.) 

 V. H. V. rhomboides var. Saxonica (Rabenh.) G. S. West [ = Navicula 

 crassinervia Breb.] is very abundant and widely distributed in the British 

 Islands, being one of the most frequent Algse in boggy districts ; length of 

 valves 50 80 /*. Pure gatherings of it can often be collected from Sphag- 

 num-pools. 



