This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Fronds small, twice to four times pinnatifid or pinnate; pinnules with a midrib and no lateral veins. Sori marginal, axillary or terminal. Involucre bivalved, of the same texture as and sunk in the frond, or free. Spore-cases sessile, on a columnar receptacle within the involucre; ring oblique. There are seventy species known, chiefly from tropical and south temperate regions. The name is from a membrane, and a leaf, from the texture of the fronds.
1. H. Tunbridgense. - Fronds 1 to 4 inches high, ovate, pinnate at the base, pinnatifid upwards; pinnules linear, undi'vided or bifid, bristly toothed. Involucre toothed. This elegant little plant grows in dense matted patches, and in habit resembles more some of the Hepaticce than the true Ferns. It is found in moist, shady situations in many parts of Britain, and throughout Europe from Belgium southwards, and also in the south temperate regions.
2. H. unilaterale, syn. H. Wilsoni. - This is very near the last, but it has recurved darker green more rigid pinnae, and entire lips to the involucres. It has about the same range as No. 1.