Trunk tall and slender, tapering from a swollen base; spathes numerous (7); inflorescence appearing in the axis of the rather persistent lower leaves, long and slender; staminate flowers arranged in rows. AERIA. Trunk robust or of uniform diameter; spathes 1 or 2; inflorescence short and brush-like, not exposed until the enclosing leaf below it falls away; flowers not set in rows. Spathe single, the fruits 2.5 cm. long; leaf-divisions upright, very dark green. ARECA. Spathes 2, fruits less than 1.25 cm. long; leaf-divisions horizontal or oblique. Trunk robust, thickened near the middle; leaf-divisions inserted by twos and standing at different angles; inflorescence twice or thrice branched, standing close to the leaf-bases. ROYSTONEA. Trunk slender, of uniform diameter; leaf-divisions at equal distances, horizontal; inflorescence once-branched, at maturity 15 cm. or more below the leaf-bases. ACRISTA. =Aeria= gen. nov. A tall slender palm evidently related to _Gaussia_, but the embryo lateral instead of basal, and the pinnae without basal cushions. Among palms in Puerto Rico _Aeria_ resembles only _Acrista_, from which it is readily distinguishable by the very slender habit, the swollen base of the trunk, the much-branched slender interfoliar inflorescence, the shorter sheathing bases of the leaves, and the numerous spathes. The embryo of _Aeria_ is located near the longitudinal middle of the seed on the side opposite the rudiment of the style, which is here located at the base of the fruit instead of on the side as in _Acrista_. The albumen is also uniform, except for a small central cavity and the outer covering is fleshy rather than fibrous. The position of the embryo is, perhaps, the most obvious difference between this genus and _Gaussia_, but there are several other significant discrepancies. Thus the flowers are arranged 3 or 4 in a row, very seldom 5 or 6. Three fruits develop from one flower only exceptionally. The trunk is of more than medium height, and the inflorescence is in reality infrafoliar, for although the dead leaf-bases and midribs of the leaves are persistent and support the long inflorescence, this condition is not comparable to that of the cocoid and other really interfoliar inflorescences. =Aeria attenuata= sp. nov. Plate 45.