14 Oephalozia. 



It may be asked why I have given such extension to Cephalozia as 

 to include in it certain groups hitherto regarded distinct genera. My 

 idea of a genus is that it should be (wherever possible) a large assem- 

 blage of closely-related species. In some cases, either because several 

 species remain still undiscovered, or because many intermediate forms . 

 have fallen out of existence — presumably from their unfitness to survive 

 — a genus may be well marked off from its fellows, although it consist 

 of but a few, or even of a solitary, species. For instance, every one 

 will recognise what a wide gap exists between such genera as Radula, 

 Scalia (= HaplomitHum ) and Aneura and all their coordinates ; and how 

 difficult it is to assign the nearest ally — or indeed any very near allies — 

 to any one of these genera. But in some tribes, of which the Trigo- 

 nanthea are an example, the wealth of existing and known species may 

 be so great, and so many forms may be nearly conterminous, that the 

 great mass of the species must necessarily be combined into genera 

 resting each on a broad base : e. gr. Cephalozia, Lepidozia, Bazzania, (s. 

 Mastygohryum), &c., &c. Even so, certain small groups may still (for 

 lack of material) remain incombinable with the larger genera, although 

 tlie acquisition of new species may enable us hereafter to effect a broader 

 synthesis. 



For my own part it is indifferent whether my sections of Cephalozia 

 be looked on as mere subgenera, which is my own view ; or that some 

 (or all) of them should be held distinct genera. On either view, it will 

 be necessary to recognise their close relationship, which is what I 

 mainly argue for. 



Wide-embracing as is the area of Cephalozia, it yet excludes a few 

 species which Dumortier, or others, have heretofore assigned to that 

 genus. One of them is the Jungermania albescens of Hooker (and its var. 

 J. Islandica Nees.) which, by its truly lateral and subpinnate ramifica- 

 tion — without a single postical branch — and by some other of its 

 characters, including even its blueish-white colour when dry, is perhaps 

 as nearly aUied to Lepidozia reptans and to Anthelia as to Cephalozia. 

 I have therefore separated it as a new genus under the name Pleuro- 

 clada.* 



*This genus, and a few others, closely related to Cephalozia, will be treated of 

 more fully in the sequel, 



