ON' GENERA AND SPECIES. 31 



sporangiferous receptacle, differences which, with two or 

 three exceptions, appear to me to be more useful as specific 

 than as generic characters ; indeed, in some cases they are 

 so trifling that I have failed to find them. Under these 

 circumstances I cannot see that any practical advantage 

 will be gained by dividing Hymenophyllacece into so many 

 genera as Presl has done. 



Hymenophyllacece also form the subject of a special mono- 

 graph, published in 1858 by Dr. Van den Bosch, of Goes, 

 Holland. It consists of seventy-nine pages octavo, and is 

 entitled " Synopsis Hymenophyllacearum, Monographic 

 hujus ordinis Prodromus." It contains simply an enu- 

 meration of names of genera and species with references to 

 authors. He enumerates no less than 305 species, being 

 120 above that of Presl, and 156 above that enumerated 

 by Sir William Hooker in the " Synopsis Filicum," here- 

 after to be noticed. The whole are arranged under nine 

 genera, of which Hymenophyllum and Trichomanes alone 

 contain 254 species. The remaining fifty-one are divided 

 amongst seven genera, two of which contain a single 

 species each. He does not characterise any new genera, 

 and only adopts five of Presl's. But his number of species 

 is so much in excess of that of other authors that, if he 

 really had specimens of the whole in his herbarium he 

 must have described his species on the different forms of 

 fronds and other variable structure, which are always to 

 be found in an extensive series of herbarium specimens, 

 many such when carefully collated represent a single 

 species only. 



In 1849, Presl published another work, entitled " Epimelia 

 Botanica," or additions to and emendations of his " Tenta- 

 men Pteridographeas." In this work he characterised no 

 less than thirty-five new genera, and described a consider- 



