 There are only three living genera in the family Marsileaceae . The majority of species ( about 45 to 70 ) belong to the genus Marsilea , which grows worldwide in warm @-@ temperate and tropical regions . Marsilea can be distinguished from the other two genera by the presence of four leaflets on each leaf , although some species occasionally produce six leaflets per leaf . A second genus Regnellidium includes a single living species that grows only in southern Brazil and neighboring parts of Argentina ; it has only two leaflets per leaf . The third genus Pilularia grows widely in temperate regions of both the northern and southern hemispheres . Its leaves do not subdivide into leaflets but are slender and tapered to a point , so that it is often overlooked and mistaken for a grass . There are only about five species known . 
