species q/'Didymograpsus. 355 



five lines each in length, with an average breadth of about 

 half a line, diverging from a mucronate base at an angle of 

 about 60°. The base is rounded, and is seen, in the few 

 specimens which are well preserved, to be provided with two 



Fig. 9. 



Didymograpsiis sextans : a, a specimen slightly enlarged and with the 

 cellules partially restored ; b, base of the same, enlarged. 



lateral spines, and sometimes with a central minute spine or 

 radicle, though this latter can only rarely be detected. The 

 radicle is, as usual, on the inferior aspect of the frond, and the 

 cellules are situated on the same side — a peculiarity found in 

 no other Didymograpsiis except D. divaricatus^ Hall. The 

 " angle of divergence " is therefore included between the non- 

 celluliferous margins of the stipes ; and it is almost always 

 about 60°. The " radicular angle " is bounded by the celluli- 

 ferous margins of the stipes, and is, of course, about 300°. 

 The cellules are from thirty to thirty-five in the space of an 

 inch, and the first two are coalcscent by their bases, as in D. 

 anceps. In all essential respects the cellules are identical with 

 those of D. divaricatus and D. anceps. The outer cell-walls, 

 namely, are curved and subparallel with the axis ; the denti- 

 cles are obtusely rounded off; and the cell-apertures form 

 oblique indentations extending about halfway across the stipe. 

 These, at any rate, are the characters of the cellules in our 

 British specimens, in those few examples in which they admit 

 of examination, as they rarely do. In Hall's original descrip- 

 tion the cellules are said to terminate in " slender mucronate 

 points ;" but some error must undoubtedly have been made 

 upon this head. This is rendered certain by the fact that Hall 

 has subsequently placed D. sextans in the gQniia Dicranogy-apsus 

 along with D. divaricatus, expressly upon the ground of the 

 similarity in the shape of the cellules, whilst he has figured 

 the latter with cellules such as I have described above. 



The propriety of placing I), sextans in the genus Dicrano- 

 grapsus as this genus is understood by British paleeontologists, 

 may still be looked upon as an open question. In none of the 

 many specimens which have passed through my hands have I 

 observed any thing more than the coalescence of the first two 

 cellules by their bases. This, though perhaps an approxima- 



