These studies were undertaken to determine the distribution patterns of the genera of living Hepaticae, and to ascertain whether these patterns might also reflect something of the past developmental history of the group and of the, vegetation of the earth. Already studies by Muller (1912-16), Domin (1923), Evans (1934), Herzog (1926) and others have indicated that the areas of distribution of individual species of Hepaticae are as sharply delimited and present. as varied patterns of distribution as do many of the Phanerogams. The species of Hepaticaedo not, therefore, show a uniformly cosmopolitan distribution, a characteristic which has been erroneously assigned to them and to other groups of spore-bearing plants. The Hepaticae comprise a group of perhaps 8,500 species,3 both leafy and thallose forms. Of these, the leafy forms constitute 7,200 species or a,bout 85%. They have been classified into 18 families (see Evans, 1939, for 'the detailed list), but it may be that this is too conservative an interpretation, particularly among those groups which represent the more primitive types. There are at present approximat'ely 180 genera of leafy forms. The relatively small' number of Hepaticae (leafy and thallose forms), and the fact that they as a group are very old, would seem to make them promising materials for research relating to problems of plant distribution. A distribution map was made for each genus from all the published rec6rds. In this type of study the species concept held by an author would have little effect on the distribution pattern of a genus; whether one recognizes 15 species or only 5 species, the over-all distribution based on those specimens is the same. At the present time this is the, only type of approach practical in the study of the distribution of' most of the larger genera outside of North Temperate areas, particularly of the tropics, since most of' the genera outside of 'the North Temperate Zone are in need of systematic study and necessarily of revision. For this reason it is not justifiable to make a study of 1 A part of the data in this paper was presented to the Section on Cryptogams, 7th International Botanical Congress, Stockholm, Sweden, in July, 1950. 2 I wish to acknowledge research funds from the Ohio Academy of Sciences and from the Graduate School of Arts and Sciences, University of Cincinnati, for use in this project; I also wish to acknowledge the assistance of Mr. Robert Boller in the making of more than 500 maps. To Dr. E. Lucy Braun, Dr. W. H. Camp, Dr. K. Caster, and Dr. J. H. Hoskins, who have made suggestions and criticisms, I am particularly indebted. 3 The classification of the Hepaticae followed here is that of Evans (1939). There are four orders: 1) Jungerfnanniales, which includes the Haplomitrineae (leafy plants in 2 genera with 5 species-the Calobryales of some authors), the Jungermanniales Acrogynae (leafy plants in approximately 180 genera and some 7,200 species -the Jungermanniales of some authors), and the Jungermanniales A-nacrogynae (thallus plants in about 22 genera containing 550 species -the Metzgeriales of some authors); 2)Marchantiales (with approximately 400 species in 32 genera, all thalloid); 3) Sphaerocarpales (with approximately 20 species in 3 genera, all thalloid); and 4) Anthocerotales (with approximately 320 species in 5, genera, all thalloid). The radial leafy forms come first and are followed by the dorsi-ventral leafy. forms and then the thallose forms. For a detailed survey of liverworts and, a discussion of the characteristics of these subdivisions and the relationships of the Hepaticae, the reader is referred to Castle (1946).