III. DESCRIPTION OF SPECIES 



Extreme types of Riccardia, such as R. pingiiis and R. Jiymcn- 

 ophylloidcs, are so distinct that they might properly be separated 

 generically, if they were not connected by intermediate types. 

 The latter, which are very numerous, do not form a continuous 

 series in which a gradual increase in complexity is exhibited. It 

 has been shown, on the contrary, that complexity in certain fea- 

 tures may be combined with simplicity in others, and combinations 

 of this sort occur in the greatest variety. This condition makes it 

 exceedingly difficult to arrange the species of the genus in natural 

 groups, and nothing is attempted in the present paper except to 

 place the more complex species at the beginning of the series and 

 the less complex at the end. The key is artificial in character and 

 is based entirely on vegetative features. In the citation of 

 specimens the following abbreviations are used : B., Boissier Her- 

 barium, at the University of Geneva ; H., Cryptogamic Herba- 

 rium of Harvard University; M., Mitten Herbarium, at the New 

 York Botanical Garden; Massal., collection of Professor Mas- 

 salongo, at Verona ; N. Y., Herbarium of the New York Botani- 

 cal Garden; S., collection of Professor Schiffner, at Vienna; 

 St., Herbarium of the Swedish Academy of Sciences, at Stock- 

 holm ; U., Herbarium of Upsala University ; U. S., United 

 States National Plerbarium; Y., Plerbarium of Yale University 

 (including the private collection of the author). 



Key to the species 



a. Main axis of thallus and its branches clearly differentiated and 

 rarely intergrading h. 



a. Main axis of thallus and its branches slightly (or not at all) 



differentiated and often intergrading /. 



h. Thallus with numerous ventral longitudinal lamellae; main 



axis with a unistratose wing i. R. fuegiensis (p. 113). 



b. Thallus without ventral lamellae ; main axis without a inii- 



stratose wing c. 



c. Main axis subterete, at least in the lower part, the sides 



rounded in cross section d. 



c. Main axis distinctly flattened throughout, the sides acute to 



acuminate in cross section e. 



