Jensenia is a dioicous, dendroid liverwort which superficially resembles some species of Pallavicinia. In fact, Jensenia is sometimes regarded as but a subgenus of Pallavicinia. However, several morphological features viewed with SEM clearly distinguish Jensenia from all other genera of the Pallaviciniaceae. This study looks at representatives of the twelve species of Jensenia, focusing on those unique features that separate it from Pallavicinia. Preliminary investigations of Jensenia erythropus, from Venezuela, reveal that the thallus arises from a dark red to purplish, underground rhizome that is covered with unicellular rhizoids. The thallus wing margin is devoid of slime hairs and the apices are deeply incised in male and female plants. Optical microscopy shows there is a prominent midrib that is one-third to one-half the thallus width that dichotomously branches in correlation with thallus furcations. The antheridia are reddish and wholly cover the midrib, unlike Pallavicinia where they are in parallel rows on both sides of the midrib. The archegonia are restricted to the dorsal surface at the basalmost furcation, which is different from the scattered arrangement seen in Pallavicinia, and are surrounded by a slightly elevated, laciniate perichaetium. The pseudoperianth, which may be more aptly called a caulocalyx, is long and cylindrical. The spores are approximately 25-40Ám in diameter with a broadly pilate wall anastomosing at the pilae heads. The thick-banded elaters are bispiral and extremely long at ca. 300Ám. At maturity the ellipsoid capsule appears to have a bistratose wall, becoming tri-to multi-stratose at the apex.

Key words: liverwort, morphological features, optical microscopy, SEM