NAVICULA AFFINIS. 33 
Navicula affinis, Ehrenberg. 
Navicula affinis, Ehren. Amer. 1843, p. 129, t. I. ii. fig. 7, IT. iii. fig. 2, 
II. iv. fig. 4, II. v. fig. 4, IIL. iii. fig. 8, IV. ii. fig. 6, and IV. 
v. fig. 10; Kiitz. Bac. p. 95, t. xxviii. fig. 65, and t. xxx. figs. 
45 & 46; Sm. Synop. vol. i. p. 50, pl. xvi. fig. 143; Ralfs, in 
Prit. Inf. p. 902, pl. xii. fig. 82; Rabenh. Siissw. Diat. p. 40, 
t. vi. fig. 58, and Europ. Diat. p. 196. 
Navicula ampliata, Ehren. Mikrogeo. t. xv. figs. 832 & 35. 
Navicula hiber, var. 8, Roper, Quart. Micr. Journ. vol. vi. p. 25, pl. iii. 
fig. 10. 
Plate V. fig. 8. 
V. linear, or linear-elliptical, with a gently incurved con- 
striction beneath the rounded extremities; striz fine, 
about 46 in :001", transverse, reaching to the median 
line, shortened opposite the central nodule, and inter- 
rupted by a longitudinal line near to and parallel 
with the-margin. 
Distinguished from N. firma by its much smaller size, 
incurved constriction near the apices, and finer striz; this 
species is not subject to much variation. I cannot agree 
with Rabenhorst that it is the type to which N. firma and 
N. amphirhynchus should be referred as varieties, although 
these species and N. Jridis are closely allied, and present 
a great similarity in the structure of their valves. 
Hab. Fresh water: common. 
Subsection V. Valve suddenly constricted into produced, capitate, 
subcapitate, linear, or mucronate extremities. 
The species included in this subsection are readily 
distinguished from those of Subsection IT. by the sudden, 
and generally deep, constriction of the valve into produced 
extremities ; in the latter the constriction is always gradual 
and tapering and in some instances very slight. Both sub- 
sections form very natural groups; and it is essential for 
