to their relation to living species, by the absence of fructifications, which essentially serve as characters for their classification. In the fossil specimens, the nervation, which is generally well preserved and distinct, together with the form of the fronds and of their divisions, the pinnae and pinnules or leaflets, are the only appreciable characters for the determination and even the general classification; as for example that of Brongniart, the oldest, simplest, and most comprehensible of those which have been attempted. It admits four general divisions:
Fig. 2.—1. Neuropteris hirsuta. 2. Adiantites Lindseæformis.
1. Neuropterideæ. Frond simple or with compound leaflets free or adhering, without middle nerve or merely with a middle nerve at the base, disappearing upward; veins dichotomous or flagellate. 2. Adiantideæ. Frond pinnate, bipinnate, or tripinnate; leaflets narrowed to the base, flabelliform, entire or scarcely lobed; veins diverging from the base, without a distinct middle nerve.
Fig. 3.—1. Sphenopteris tridactylitis. 2. Leaf enlarged.
3. Sphenopterideæ. Fronds of the same kind; veins pinnate or bipinnate near the base; secondary divisions very oblique.
Fig. 4.—Pecopteris louchitica.
4. Pecopterideæ. Fronds simple, pinnate or bipinnate, the leaflets generally adhering by the whole base, or often joined (confluent) at or near the base, then forming lobes, entire or denticulate; secondary veins pinnate, dichotomous or reticulate.—This general classification is still used as a commodious frame for establishing a large number of subdivisions, though many other systems have been proposed since. Among others, the two more important are that of
