34 One Thousand Objects 



simple; more or less rectangular in the side view, with a 

 longitudinal clear space or line down the middle, and a 

 nodule at the centre and at each extremity. The valves 

 in this species are nearly quadrangular, and the striae very 

 delicate. (PI. IV., fig. 8.) 



240. CONVEX NAVICULA (Navicula serians). Valves 

 similar in size and form to the last, except that the sides 

 are rounded, and not at all angular. The longitudinal 

 strise distinct, generally associated with the last species, 

 forming a film on the surface of boggy pools. 



241. NIPPLED NAVICULA (Navicula crassinervia). 

 Valve between elliptical and spear-shaped, with the ex- 

 tremities lengthened or produced into a kind of seed-like 

 termination. Striae delicate. Similar habitats to the pre- 

 ceding. 



242. HORNED NAVICULA (Navicula cuspidatd). Valve 

 spear-shaped, with somewhat acute extremities. The striae 

 parallel and distinct. There is a great similarity in the 

 outline of these four species, especially to the unpractised 

 eye. Of frequent occurrence in ditches. 



243. SWOLLEN NAVICULA (Navicula gibberula}. The 

 valves in this species have rounded ends, and the sides 

 have a convex inflation at the centre. Common in ditches. 



244. BORDERED NAVICULA (Navicula firma). Valve 

 narrow, gradually tapering to the rounded ends; a longi- 

 tudinal line near each margin. Central nodule Con- 

 spicuous. Stria fine. Common in ditches. 



245. CONSTRICTED NAVICULA (Navicula amphisboena). 

 Valve elliptical, with constrictions near the extremities, 

 and rounded ends. Common in fresh and brackish water. 

 (PI. IV., fig. 32.) 



246. GREAT PINNULARIA (Pinnularia major}. From 

 Navicula, this genus differs chiefly in the valves being 

 ribbed instead of striated. This is a large and fine species, 

 with the sides parallel and straight, except a slight mar- 

 ginal inflation at the centre, and blunt rounded ends. 

 The central nodule large. Marshes and sluggish streams. 



247. NARROW PINNULARIA (Pinnularia viridis}. Valve 

 narrowly elliptical, with the ends scarcely so rounded as 



