30 NAVICULER. 
Navicula dubia, Ehrenberg. 
Navicula dubia, Ehren. Amer. 1848, p. 180, t. II. ii. fig. 8; Kiitz. Bac. 
p. 96, t. xxviii. fig. 61; Rabenh. Siissw. Diat. p. 40, t. vi. 
fig. 60; Greg. Quart. Micr. Journ. vol. iv. p. 3, pl. i. fig. 3; 
Ralfs, in Prit. Inf. p. 902. 
Navicula Peisonis, Grunow, in Wien. Verh. 1860, p. 544, t. i. fig. 28; 
Rabenh. Europ. Diat. p. 202. 
Plate V. fig. 5. 
Frustule small. V. linear, with slightly produced, narrow, 
obtuse, or subacute, extremities; margins straight, or 
slightly triundulate; striz fine, delicate, transverse, 
reaching to the median line and crossed, near the 
margin, by a longitudinal line. 
Distinguished from N. affinis by its much smaller size, 
finer striz, and suddenly produced, narrow extremities. 
Hab. Fresh water. Elgin and Loch Leven, Scotland 
(Prof. Gregory); River Coquet Northumberland (A. 8. D.). 
Navicula Iridis, Ehrenberg. 
Navicula Tridis, Ehren, Amer. 1843, p. 180, t. IV. i. fig. 2; Kiitz. Bac. 
p. 92, t. xxviii. fig. 42; Ralfs, in Prit. Inf. p. 907; Rabenh. 
Europ. Diat. p. 171. 
Pinnularia Iridis, Rabenh. Siissw. Diat. p. 42, t. vi. fig. 1. 
Navicula firma, Sm. Synop. vol. i. p. 48, pl. xvi. fig. 188. 
Plate V. fig. 6. 
Frustule large, on M.V. linear, with rounded. extremities. 
V. linear or linear-elliptical, with rounded extremities 
and conspicuous terminal nodules; striz fine, about 
35 in ‘001", transverse, reaching to the median line, 
shortened opposite the central nodule, and interrupted 
near the margin by a series of wavy longitudinal 
lines. Colour brownish, iridescent. 
This is a distinct and well-marked species, although by 
