NICHOLSON SKIDDAW GRAPTOLITES. 133 



and I have obtained excellent specimens, in situ, from some of the 

 highest beds at Ellergill, near Millburn. 



Loc. BarfF; Outerside ; Whiteside, near Crummock ; Skiddaw 

 Longside ; Ellergill, in Westmoreland. 



8. Phtllograpsus typtjs. Hall. PI. V. fig. 16. (Grapt. Quebec 

 group, p. 119, pl. 15. figs. 1-12.) 



This form differs from the above chiefly in the shape of the frond, 

 which is " elongato-ovate or lanceolate, broad-oval or obovate," 

 and likewise in the fact that the apertures of a third row of cellules 

 are always exhibited. The shape and size of the frond, however, 

 are very variable, and too much stress should not be laid upon 

 this character alone. The cellules are about 24 in an inch, the 

 apertures being mucronate, though the mucrones are not so pro- 

 nounced as in P. angiistifolius. 



Loc. Barff, near Keswick ; and Skiddaw Longside. 



Genus Didtmograpsfs, McCoy. 



This genus contains those Graptolites in which the frond con- 

 sists of two simple monoprionidian stipes, diverging from a mucro- 

 nate "initial point" or "radicle." The radicle is occasionally 

 rudimentary, as in D. sextans, HaU, and D. hifidus, Hall (in some 

 cases), or is even apparently absent, as in most specimens of D. 

 anceps, Nich. The genus Didymograpsus, beyond aU doubt, in- 

 cludes a series of perfect and unbroken forms, which never, at any 

 time of their existence, constituted factors of compound organisms ; 

 though Hall adopts this latter view, and includes the whole group 

 in his genus Grajjtolithus. It cannot, however, be questioned 

 that such compound forms would have been discovered, had they 

 really existed, amongst the thousands of specimens which have been 

 exhumed from the Silurian rocks of Scotland and Wales ; and the 

 absence of any traces of such is ample proof that Hall's conjecture 

 is without sufficient foundation. 



Seven species of Didymograpsus are known to me as occurring 

 in the Skiddaw Slates, of which three are new to Britain. Of the 

 entire number, three are confined to this horizon, viz. D. V.-fractus, 

 Salt., D. nitidus, Hall, and D. hifidus. Hall ; two are well-known 

 Lower-Llandeilo forms, viz. D. geminus. His., and D. patulus. Hall 

 (=Z). hirundo. Salt.) ; whilst two are common to the Upper Llan- 

 deilo and Caradoc rocks, viz. D. serratulus, Hall, and I), sextans, 

 Hall. 



9. Didymograpsus cabucefs, Salter. 



This was originally described by Mr. Salter, from Canadian speci- 

 mens, as a distinct species (Quart. Journ. Geol. Soc. vol. ix. p. 87), 

 and was afterwards noticed by him from the Skiddaw Slates (ibid. 

 vol. xix. p. 138). It was, however, subsequently rejected by Hall, 

 on the ground that the species had been founded on specimens of 

 Tetragrajpsus hryonoides, HaU, in which two only of the sdpes 



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