344 Dr. II. A. Nicholson on the British 



ceeding from a long and slender radicle and including between 

 them an angle of (livcrgence which may be stated to average 

 140°. If I am right, however, in referring to this sjx'cies a 

 number of ill-preserved forms whicli occur in tlie Skiddaw 

 Slates, the angle of divergence is exceedingly variable, ranging 

 from no more than 80° up to very nearly 180°. In the tigures 

 which I have given of these Skiddaw-Slate specimens, fig. 3 

 may be taken as the typical form ; and there can be no doubt 

 of the identity of this with HalTs species. Fig. 3 c shows a 

 form apparently the same in all essential characters, but having 

 an angle of divergence of close upon 180°, whilst fig. Sd ex- 

 hibits a very much smaller angle, but is in other respects the 

 same. The preservation, however, of these forms is so bad 

 that it is impossible to be positive as to their absolute 

 identity. 



In all these cases we have the following common characters, 

 when the state of preservation is such as to allow of tlu'ir 

 determination : — 



The stipes are exceedingly slender, from one-fortieth to one- 

 thirtieth of an inch at their commencement, and they widen 

 out very slowly, never attaining a greater width than from 

 one twenty-fourth to one-twentieth of an inch. The length of 

 the stipes is very great, being over four inches in one sj)eci- 

 men. In the most typical forms the stipes are perfectly 

 straight, but in others they are gently curved. The cellules 

 are always on the opposite side of the frond to the radicle, or 

 occupy the sides of the angle of divergence. They vary in 

 number from twenty-five to more than thirty in the space of 

 an inch ; they make a small angle with the axis ; and the 

 cell-mouths are at rigiit angles to the axis, giving the frag- 

 ments a close supcriicial resemblance to G. sayittariiis. The 

 radicle is always very long and slender. 



The only Skiddaw-Slate species with Avhich these could be 

 confounded is D. extensiis ; but the radicle in this species 

 appears to be always short and blunt, and the stipes attain a 

 decidedly greater width, whilst the angle of divergence is 

 constantly 180°. The preservation of the specimens here 

 referred to D. serratulus is too poor to allow of any more mi- 

 nute comparison. 



Loc. Skiddaw Slates (lower beds), Outerside and BarfF, 

 near Keswick ; (upper beds) Thornship Beck, near Shap. 



Didymograpsiis fasciculatuSj Nich. Fig. 5. 

 (Ann. & Mag. Nat. Hist. October 1869, pi. 11. figs. 21, 22.) 

 Frond consisting of two simple stipes arising from a short 



