NECKER'S GENERA OF FERNS. 105 
Necker's fern-genera are nonentities, not represented by 
types, nor to be recognized with certainty.! 
There is another of our author's fern-genera more eaaily 
recognizable, if possible, than Achomanes, and that is CErmosrs. 
It is taken, Necker is careful to tell us, out of the simple- 
fronded Pteris species of Linneus. The Linnean group 
placed under the caption “Frondibus simplicissimis” is made 
up of the four species, P. lanceolata, lineata, tricuspidata and 
furcata. Now, out of Necker’s indicative note ''Frondes 
simplices. Qued. Pterid. Linn.” alone, we can make 
nothing more definite than that some one or more of 
these constitute his (tosis. But we have not yet read 
his generic diagnosis, or even that part of it which 
according to the terminology of Linnzeus was called the 
character essentialis, and which with Necker is termed the 
character peculiaris. Now this particular and decisive mark 
of his Gnosis Necker says is, that the lines of the fructifi- 
cation are parallel on the frond as a whole. In this light 
the perfect identification of this genus should be as easy as 
the distinguishing of two parallel lines from two converg- 
ing ones. In order that any frond may have its marginal 
fruiting lines parallel, it must not only be a simple frond 
but a linear oue. P. furcata can not be a member of tosis 
because its fruit-lines converge in pairs, each pair forming 
two sides of a triangle. P. tricuspidata is equally excluded 
by the fact that, while along the linear and entire middle of 
the frond they are parallel, they take the very extreme of a 
zigzag course across the cleft frondal apex. P. lanceolata 
has a lanceolate frond, whose marginal fruit lines never do, 
and by the most obvious certainties of mathematies never 
ean run parallel, so long as the frond retains a lance-shaped 
outline. Only one of the four remains, and that is P. lineata. 
Its fronds are all invariably simple and linear; its fruit- 
lines are absolutely parallel; and there is no fact in all the 
! See Mem. Torr. Club, vi. 259. 
