Naviculacece 



295 



Fig. 137. A and B, Vanheurckia rhomboides (Ehrenb.) Breb., from Mickle Fell, 

 N. Yorks. (x520). C, Amphipleura pellucida Kiitz., from Chippenham Fen, 

 Cambridge ( x 520). D and E, girdle- and valve-views of Navicula viridis Kiitz., 

 to show chromatophores (ch) and nucleus (n), x 400. F, Eunotia gracilis 

 (Ehrenb.) Rabenh., girdle view to show chromatophores and nucleus ( x 400). 



Genus Amphipleura Kiitz., 1844. The frustules are solitary 

 and free-floating, elongate-fusiform in shape, with a small marginal 

 keel near each edge of the valves. The valve-view is narrowly 

 lanceolate and the central nodule is greatly elongated, forming a 

 narrow rib which separates the raphe into two short portions, each 

 portion being situated towards one extremity of the valve and 

 enclosed between two parallel ribs. These two ribs unite together 

 at each extremity of the valve to form the polar nodules. The 

 valves are transversely striated and in some species these striae 

 are exceedingly fine. 



Only one species, A. pellucida Kiitz., inhabits the freshwaters of the 

 British Isles. It is generally distributed and often abundant ; length of 

 valves 80 140/*; fig. 137 C. The striae of this species are extremely fine 

 (about 37 in 10 p.) and the valves are used as microscopic test-objects. Some 

 very large forms of it sometimes occur in the freshwater plankton. 



Genus Gyrosigma Hassall, 1845. [Pleurosigma W. Sm., 1853; 

 ? Scalprum Corda, 1835.] The frustules are elongated, of the 

 naviculoid . type, and the valves are convex. In the valve- view 

 they are sigmoid with obtuse or attenuated extremities. The 

 raphe is also sigmoid. There are two sets of striations on the 

 valves, which cross one another either at right angles or obliquely. 

 The girdle-view is generally straight and linear-oblong in shape. 

 The chromatophores consist of a pair of large jagged or perforated 



