340 Dr. H. A. Nicholson on the British 



Didymograpsns hirmido, Salt. (Quart. Journ. Geol. Soc. vol. xix. p. 137, 

 tig. 13/; Mem. Geol. Survey, vol. iii. p. 331 and pi. 11. figs. & 7). 



Didymoifrapsus liatulus (Nicholson, Quart. Journ. Geol. Soc. vol. xxiv. 

 p. 135). 



Frond composed of two monoprionidian stipes diverging- 

 from a small radicle at an angle of 180° (sometimes a little 

 less, and very rarely a little more). The stipes have a very 

 considerable length, reaching two or three inches each with- 

 out showing any signs of a termination. The stipes are 

 narrow at their commencement, but widen out gradually till a 

 width of one-tenth of an inch may be attained. In smaller 

 specimens, however, as in the subjoined cut, this width is not 



Fig. 1. 



I a 



a, Small specimen oi Didymngrapsuspcdtdus, Hall, from the Skiddaw Slates 

 of Outerside, near Keswick, nat. size ; b, fragment of D. extensus, en- 

 larged, to show the smaller inclination of the cellules. 



reached. The cellules are on the opposite side of the frond to 

 the radicle, or, in other words, they occupy the sides of the 

 angle of divergence. The number of cellules to an inch is 

 from thirty to thirty-two or thirty-four in our British speci- 

 mens, but is stated by Hall as not more than from twenty-four 

 to twenty-six in the American examples. The cellules make 

 with the axis an angle of between 50° and 60° ; the cell- 

 mouths make an angle of 100° to 120° with the axis, and they 

 are always produced into well-marked submucronate denticles. 

 In Hall's better-preserved specimens the outline of the cell- 

 apertures is seen to be curved, and the walls of the cellules 

 are marked with fine striai or lines of growth running parallel 

 to the cell-mouths. 



On comparing Hall's beautiful figures of this species with 

 the woodcut in Mr. Salter's above-quoted paper, there cannot 

 be any question that D. hii^undo, Salt., is the same as J), 

 patulus, Hall ; and the latter name must be retained, as it has 

 the priority. In the Memoir of the Geological Survey (vol. iii. 

 p. 331), Mr. Salter's description confirms this in every re- 

 spect. The figures 6 and 7 in pi. 11 of the same work are 

 not named, but they are apparently intended for D. hirundo. 

 If this be so, they neither conform with Mr. Salter's own de- 

 scription and previous figure of the species, nor with Hall's 

 account of D. jxitulus. It is probable, therefore, that some 

 error has crept in here, and the figures have not been intended 

 for D. hirundo. The Didyhiograpfdis figured in Lyell's ' Ele- 



