32 NAVICULEZ. 
however, had elevated into a species that form of it in 
which the linear margins and cuneate extremities are most 
prominent, and into another species the other variety, in 
which the middle of the valve is more tumid or elliptical ; 
the former he named N. amphigomphus, the latter N. dila- 
tata. Between these two forms, however, every degree of 
gradation is observable; consequently both are synony- 
mous with Kiitzing’s type. The late Prof. Smith unfor- 
tunately, in his ‘Synopsis’ (Joc. cit.) , described and figured 
N. Iridis, Ehren., as the typical N. firma, and referred 
Kiitzing’s original type to it as a variety, as well as the 
N. amphigomphus of Ehrenberg. Rabenhorst (Europ. 
Diat. loc. cit.) first refers N. firma, Kitz., to N. affnis, 
Ehren., as a var., and afterwards places it amongst the 
fossil and extra-European species, referring to it as “fossilis 
in Italia” (p. 226). Should Pinnularia amphigomphus, 
Ehren. (Mikrogeo.), on further investigation prove to be 
a species distinct from N. firma, its specific appellation 
must be retained. 
Navicula microstoma, Kitzing, is synonymous with his 
Nav. lata (Bac. p. 92, t. iii. fig. 51); and although con- 
sidered by Rabenhorst to be identical with N. firma, it is 
evidently a distinct species. It is distinguished by its 
larger size and very turgid middle tapering to the subacute 
extremities, as well as by the series of longitudinal lines 
crossing the strie being placed about midway between the 
median line and the margin. To this species should be 
referred the fine large form occurring in several diatoma- 
ceous deposits from the United States; of this Mr. Kitton 
has given a figure and description (‘Science Gossip,’ April 
1867, p. 156, fig. 158). The American specimens are very 
large and beautiful, especially those in my possession from 
the Cherryfield deposit. 
Hab. Fresh water: frequent. 
