338 Dr. H. A. Nicholson 07i the British 



The species of the genus Didymograjjsus may be conve- 

 niently and naturally divided into three sections : — 



I. Those Didymograpsi in which the radicle is on the infe- 

 rior aspect of the frond, and the cellules are on the opposite or 

 superior aspect, whilst the "angle of divergence" of the two 

 stipes is not greater than 180°. This group comprises D. 

 MurcJiisonij D. geminus, D. q^nis, D. jJatuhis^ and, in fact, 

 the greater number of the Didymograpsi. 



II. Those Didymograpsi in which the radicle is on the 

 inferior side of the frond, and the cellules on the opposite or 

 superior aspect, as before ; but the angle of divergence of the 

 stipes is now greater than 180°. In this group, which differs 

 from the last only in the fact' that the stipes are reflexed, are 

 D. Jlaccidus, Hall, and D. anceps, Nich. 



III. Those Didymograpsi in which the radicle maintains 

 its position, but the situation of the cellules is reversed, these 

 being now placed on the inferior aspect of the frond, or on the 

 same side as the radicle. In this group are D. sextans. Hall, 

 and D. divaricatus, Hall. 



For the full comprehension of the value of the above divi- 

 sions, it is necessary to define exactly what is to be understood 

 by the " angle of divergence," since this term has been very 

 loosely employed, and has led to a great deal of confusion. 

 As I have before had occasion to remark, it is perfectly ob- 

 vious that in any Didymograpsus the two stipes form two 

 angles — one upon one side of the frond, and one upon the op- 

 posite side. Each of these angles has, in different species, 

 been treated as the angle of divergence ; but it is absolutely 

 necessary to fix accurately one of these angles, which can be 

 constantly employed as a standard of comparison. In the 

 foregoing definitions, therefore, and in the following descrip- 

 tions, I shall employ the term " angle of divergence " solely 

 for the angle included between the stipes on the side of the 

 frond opposite to that on which the radicle is situated. The 

 other angle, or the angle included between the stipes on the 

 same side as the radicle, I shall terra the " radicular angle." 

 As the radicle in all cases marks the organic base of the frond, 

 we obtain thus a constant standard of comparison between the 

 different species, however much the position of the cellules 

 may vary. 



It will, then, be at once seen, that in the first two sections 

 of Didymograp)sus, the " angle of divergence " is on the same 

 side of the frond as the cellules, or, in other words, it is the 

 angle included between the celluliferous margins of the stipes. 

 In D. sextans and D. divaricatus, however, which constitute 



