344 Dr. H. A. Nicholson on the British 



ceedinc: from a Ions: and slender radicle and includins: between 

 them an angle of divergence which may be stated to average 

 140°. If I am right, however, in referring to this species a 

 number of ill-preserved forms which occur in the Skiddaw 

 Slates, the angle of divergence is exceedingly variable, ranging 

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

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

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

 of the identity of this with Hall's 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. 3 d 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 their 

 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 speci- 

 men. In the most tj^pical forais the stipes are perfectly 

 straight, but in others they are gently cuiwed. 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-hve to more than thirty in the space of 

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

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

 ments a close superficial resemblance to G. Sagittarius. The 

 radicle is always very long and slender. 



The only Skiddaw-Slate species with which 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 BartF, 

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



Didymograpsus fasciculatus^ Nich. Fig. 5. 

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

 Frond consisting of two simple stipes arising from a short 



