NAVICULA DIDYMA. 51 
Navicula didyma, Ehren., Sm. Synop. vol. i. pl. xvii. fig. 154a. Forma 
typica, Rabenh. Europ. Diat. p. 203. New var. Greg. Trans. 
Micr. Soc. n. s, vol. iv. p. 45, pl. v. fig. 16. 
Plate VII. figs. 7a & 76. 
V. divided by a sinuous constriction into two broad, some- 
what unegual lobes, with subcuneate, or rounded, ex- 
tremities ; striz radiating, coarsely granular, about 
20 in 001"; inner section faint, longitudinal intersect- 
ing groove incurved; median line broad, tapering 
towards the terminal nodules. 
This species is very variable in size and outline, and every 
degree of gradation between the two forms figured is met 
with. 
Hab. Brackish water: common. 
+ 
Navicula didyma, Ehrenberg. 
Navicula didyma, Ehren., Kiitz. Bac. p. 100, t. iv. fig. 7, t. xxviii. fig. 75; 
Ralfs, in Prit. Inf. pl. xv. fig. 12. var. pinnis manifestis, 
Rabenh. Europ. Diat. p. 208. 
Pinndlaria didyma, Ehren. Amer. II, iv. fig. 3; Rabenh. Siissw. Diat. 
p. 46, pl. vi. fig. 26. 
Diploneis didyma, Ehren. Mikrogeo. t. xix. fig. 32. 
Plate VII. figs. 8a & 86. 
V. oblong, slightly constricted in the middle, extremities 
broadly rounded ; striz subparallel, nearly transverse, 
coarse, obscurely granular, about 17 in ‘001"; outer 
section very conspicuous, inner faint and linear. 
The species originally described and figured by Ehren- 
berg as Pinnularia didyma seems to have been lost sight 
of by recent observers, although his description as given 
by Kiitzing is sufficiently plain (striata, latiuscula, media 
