296 



Bacillariece 



plates disposed in the manner common to all the genera of the 

 family Naviculaceae. 



There are only four British freshwater species, of which G. attenuatum 

 (Ktttz.) Rabenh. (length of valves 190 250 p; fig. 138 A) and G. Spenc&rii 

 (Queck.) O. K. (length of valves 80 130 /i) are the most abundant. 



Genus Amphiprora Ehrenb., 1843 ; em. Cleve, 1891. [Amphi- 

 campa Rabenh., 1864.] The frustules 

 are free-floating and solitary, with a 

 slight twist around the longitudinal axis. 

 In the valve-view they are of a navi- 

 culoid form with a sigmoid raphe, the 

 latter being situated on a prominent 

 sigmoid keel in the sagittal axis. In 

 the girdle-view the frustules are broadly 

 inflated, with a median constriction and 

 truncate ends. The girdle itself exhibits 

 a slight twist, and seen obliquely the 

 sigmoid keels of the valves cause the 

 frustules to appear much more twisted 

 than is truly the case. The terminal no- 

 dules are not very conspicuous and there 

 is a small central nodule. The valves are 

 finely and transversely striated. 



A. paludosa W. Sm. is often found in 

 fresh water, sometimes in great abundance in 

 small ponds and ditches; length of valves 

 40 80/n; fig. 138 B and C. A. ornata Bailey 

 is also known from the freshwaters of England. 



Fig. 138. A, Gyrosigma 

 attenuatum (Kiitz.) Rabenh., 

 from Chippenham Fen, Cam- 

 bridge ( x 400). B and C, 

 Amphiprora paludosa W. Sm., 

 from Wimbledon Common, 

 Surrey ( x 400). 



Fig. 139. Mastogloia 

 Smithii Thw., from Baildon, 

 W. Yorks. ( x 500). 



Genus Mastogloia Thwaites, 1848. 

 The frustules are of a naviculoid form 

 and are enclosed in a gelatinous envelope 

 of considerable size. The valve-view 

 is usually elliptic-lanceolate, generally 

 with produced extremities, and the 

 girdle-view is sub-rectangular. Each 

 frustule possesses two longitudinal septa 

 with a large central perforation and a 

 row of marginal ones on each side. 

 These two longitudinal septa are per- 



