352 Dr. II. A. Nicholson on the British 



radicle. In some cases the initial point is recognized simply 

 by the fact that it is the point of flexion of the frond, and from 

 it the cellules point in opposite directions. In other specimens 

 the initial point is marked by the presence of a slender radicle, 

 the length of which varies from a mere node up to nearly one 

 line. In all specimens which exhibit any traces of a radicle, 

 without exception, this is on the inferior^ whilst the cellules 

 are on the superior aspect of the frond, so that the two are on 

 opposite sides (fig. 8 h). The result of this is, that the " angle 

 of divergence," properly speaking (namely, the angle formed 

 by the stipes on the opposite side of the frond to the radicle), 

 is in this case to be measured between the celluliferous mar- 

 gins of the stipes ; and it varies from 340° to 355°. The " radi- 

 cular angle," on the other hand, is included between the non- 

 celluliferous margins of the stipes ; and it varies from 5° to 20°. 

 The margin of the frond opposite to the radicle is never orna- 

 mented by spines, and is simply formed by the coalescence of 

 the bases of the first two cellules. This structure is of interest, 

 as agreeing with D. sextans^ Hall (at any rate, in its ordinary 

 form) , and apparently foreshadowing what we find in Dicrano- 

 grajysus. The stipes are very little narrower at their origin 

 than elsewhere ; and they retain a pretty uniform width through- 

 out, varying in different individuals from one twenty-fourth 

 of an inch up to two thirds of a line. The cellules are not 

 distinguishable in shape from those of D. dtvaricatus, Hall, 

 and D. sextans, Hall. They are from twenty-five to thirty in 

 the space of an inch, their outer margins convex and nearly 

 parallel to the axis, their apices rounded off, and the cell- 

 apertures forming oblique pouch-like indentations, which ex- 

 tend halfway across the stipe. In some specimens, the first 

 few cellules on either side of the initial j^oint are provided 

 each with a short blunt spine proceeding from the centre of 

 their outer margins. In some examples there are minute 

 pustules or circular depressions in the centre of each denticle 

 where it joins the body of the stipe ; but this phenomenon is 

 not constant in its occurrence. As I have already said, in the 

 shape of the cellules D. anceps is not distinguishable from D. 

 divaricatus, Hall [=D. Moffatensis, Carr. ?). In all other re- 

 spects, however, they are totally distinct ; and they could only 

 be confounded, as they have been (Carruthcrs, Geol. Mag. 

 val. V. p. 129), by turning D. anceps upside down. In the 

 first place, in D. anceps the radicle and cellules are on opposite 

 sides of the frond, whilst in D. divaricatus they are on the 

 same side. In addition to this very obvious and, indeed, 

 fundamental distinction, tlic following points of difFer,ence may 

 be mentioned : — In D. anceps the " angle of divergence," as 



