Species o/* Didjmograpsus. 343 



dantly and in a state of beautiful preservation in one locality 

 in the Skiddaw Slates. Tlie specimens from wliicli the above 

 description. is taken agree perfectly with some of Hall's figures 

 (pi. 1. figs. 1, 6, 9) ; but Hall has referred to this species other 

 examples (pi. 1. figs. 3, 7, 8) which are considerably larger, 

 and which approximate more closely to D. patulus. 

 Loc. Skiddaw Slates, Barfi", near KesAvick. 



Didymograpsus affinisj Nich. Fig. 4. 

 (Ann. & Mag. Nat. Hist. October 1869, pi. 11. fig. 20.) 



Frond composed of two simple linear stipes, of extreme 

 tenuity, proceeding from a long pointed radicle at an angle of 

 divergence of from 90° to 15()°. Tlie stipes vary in length 

 from one-half to three-quarters of 

 an inch each, and have a uniform 

 width of not more than from one- 

 fortieth to one-fiftieth of an inch, 

 which never appears to be ex- 

 ceeded. The cellules are on the 

 opposite side of the frond to the 

 radicle, or occupy the sides of tlie 

 angle of divergence. In shape a, Dkh/mof/nipsuf! affiuis, from 

 the cellules are altogether undis- the Skiddaw Slates, nat. size ; 

 tinguishable from those of G. f^/'^f™^"^ ""^ *^^ ^^°'^' ^"" 

 Nihsoni^ Barr., and they vary ^ 



from sixteen to eighteen in the space of an inch. They are 

 inclined to the axis at an extremely low angle (from 15° to 

 20°) ; they do not overlap one another at all ; and the cell- 

 mouths are from three to four times as short as the outer 

 cell-walls, and form short transverse apertin-es at right angles 

 to the axis. 



This little species occurs in great numbers, all confusedly 

 matted together, in some parts of the Skiddaw Slates, it being- 

 rare to find a detached individual showing both sides of the 

 frond. The characters of the cellules are alone quite sufficient 

 to separate the species from all other known forms. 



Loc. Lower beds of the Skiddaw Slates, Bai-ff, near Kes- 

 wick ; upper beds of the Skiddaw Slates, Ellergill, near Mil- 

 bum, and Eggbeck, near Pooley. 



Didymograpsus serratuhrs, Hall, sp. PI. YIT. figs. 3, 

 3 (7, 3 h, 3 r, 3 d. 



Graptolithus serrafulus, Hall (Pal. N. York, vol. i. p. 274, pi. 74. fig'. .5). 

 Didymoqrapsus serratuhis (Nicholson, Quart. .Tourn. freol. Soc. vol. xxiv. 

 p. 136). 



Frond comijosed of two long and vew slender stipes pru- 



24* 



