II MUSCINE^— HEPATIC^— MARCHANTIACEJE 41 



the floating form. The ventral scales become shorter, and 

 numerous wide but unthickened root-hairs are formed, which are 

 almost completely lacking in the floating form. The structure 

 of the reproductive organs and sporogonium are essentially the 

 same as in Riccia, except that the plants are strictly dioecious. 



The third genus, Tesselina {Oxymitra), represented by the 

 single species, T. pp'ainidata, is much less widely distributed, 

 belonging mainly to Southern Europe, but also found in 

 Paraguay. This interesting form has also been carefully 

 examined by Leitgeb,^ who calls attention to its intermediate 

 position between the Ricciaceae and the Marchantieae. The 

 thallus has all the characters of the latter : air-chambers opening 

 by regular pores, usually surrounded by six guard-cells; two rows 

 of ventral scales, independent from the beginning ; and the 

 sexual organs united into groups upon special parts of the 

 thallus. The sporogonium, however, is entirely like that of 

 Riccia, so that it may properly be placed in the same family. 

 The plants are dioecious and strictly terrestrial. 



A third genus, Cronisia, represented also by a single species, 

 C. paradoxa, is placed provisionally with the Ricciaceae by 

 Schiffner,^ but the structure and development have not been 

 investigated with sufficient completeness to make this certain. 

 It has been found only in Brazil. Schiffner says of this form : 

 " It belongs perhaps to the Corsinieae, and forms a direct 

 transition from the Ricciacea: to that family." 



^ Leitgeb (7), vol. iv. p. 34. - Schiffner (i), p. iS- 



