The mature vegetative organs are described briefly. Tlhe development and mature structure of the leaf and sporocarp are given in some detail and these are compared with these same features in Marsilea quadrifolia, Regnellidium diphyllum, and Pilularia globulifera. The early development of the leaf and that of the sporocarp of Pilularia minuta is closely like that of the other Marsileaceae thus far studied. The sporocarp of this Pilularia is clearly dorsi-ventral in symmetry, both in external form and in details of development of the capsule, including its highly specialized wall. The whole plan of this capsule is like that of a Marsilea or of P. globulifera except that the number of sori in P. minuta is reduced to 2. This structure of its capsule as well as that of its vegetative organs show P. minuta to be the simplest of all the Marsileaceae.