Sj)ecies o/'Didymograpsiis. 



347 



towards the extremity. The ceUuliferous margin of each stipe 

 is therefore curved, whilst the back is more or less completely 

 straight. The cellules are placed on the opposite side of the 

 frond to the radicle, or occupy the sides of the angle of diver- 

 gence. The cellules are from thirty-two to thirty-six in the 

 space of an inch, long, narrow, and slightly curved, inclined 

 to the axis at an angle of about 45°, the cell-mouths curved 

 and prolonged into long submucronate teeth. The base is 



Fig. 7. 



Didymograpsus brftfJus, from the Skiddaw Slates : a, typical example, 

 natural size; b, fragment of the same, enlarged, to show the cellules; 

 c, base of another individual, with a well-developed radicle ; d, a small 

 example hardly separable from D. Murchisoni, slightly enlarged ; c, base 

 of another example, in which the radicle is quite rudimentary. 



usually rounded, with a short obtuse radicle; but in some cases 

 it is much more pointed, and the radicle is pretty long. 



In its most typical form (as in fig. la) the distinctness of this 

 species can hardly be a matter of question. The smaller forms, 

 however, of D. htjidus, and especially those which have a 

 pointed base and a well-developed radicle, are certainly not 

 distinguishable by any good characters from the younger ex- 

 amples of J). Murchisoni. This latter form, however, has 

 hitherto proved so local in its distribution, and the fully grown 

 forms of the two species are so distinct, that I prefer retaining 

 all my Skiddaw-Slate specimens, in the meanwhile, under D. 

 hijidus. 



Loc. Upper beds of the Skiddaw Slates : Ellergill, near 

 Milburn (abundant and very Avell preserved) ; Eggbeck, near 

 Pooley. Rare in the lower beds of the Skiddaw Slates : Outer- 

 side, near Keswick. 



Of the nine species of Didymofp-apsus which I have now 

 described as occurring in the Skiddaw Slates, it will be seen 

 that all, except D.fasciculatus^ belong to the first section of 

 the Didi/mograpsi — namely, to those in which the cellules are 

 on the opposite side of the frond to the radicle, and the angle 

 of divergence is not more than 180°. Indeed D. fasciculatas 



