28 BRITISH GRAPTOLITES. 



Affinities, — D. Nicholsoni may be said to be characterised — 



(1) by the rigidity of the stipes, 



(2) by the characters of the thecse : 



(a) their low angle of inclination, 



(h) the number of a given unit of length. 

 From D. nffinis, to which it is closely allied, it may be distinguished — 



(1) by the greater width of the stipes, 



(2) the characters of the thecse : 



{(i) their greater overlap, 



(b) higher inclination, and 



(c) greater nmnber in a given unit of length. 



It has been already pointed out (' Quart. Journ. Geol. Soc.,' vol. liv, p. 502) 

 that the Skiddaw Slate specimens were provisionally referred by Nicholson to D. 

 serratidus, from which, however, he considered they varied in some particulars. 

 Lapworth (see synonymy) showed that the thecas differed in cei'tain important 

 features from those of Hall's species, and additional characteristics afforded by 

 specimens subsequently discovered in the St. David's district convinced him that 

 the present form was worthy of being separated as a distinct species, for which he 

 proposed the name D. Nicholsoni. 



Horizon and Localities. — Arenig (r), Upper part (Middle and Upper SkiddaAv 

 Slates) ; Llanvirn (St. David's) ; Upper Arenig of Shelve. 



Lake District : Barf, near Keswick ; Carlside Edge, Skiddaw ; Outerside ; 

 Thornship Beck, Shap. F)t. DaricVs District: Porth-hayog, Ramsey Island; 

 Llanvirn Quarry; Whitland, Pwllacca. Shwj^sliire : Ritton Castle, Shelve. 



Associates, etc. — The species occurs low down in the Skiddaw Slates associated 

 witli Tetragraptns qiiadribrachiatus, and also in somewhat higher beds at 

 Outerside and Barf; at St. David's it occurs exclusively — so far as we at 

 present know — in beds corresponding with the higher beds of the Skiddaw 

 series of the Lake district, namely, those of Llanvirn, where it occurs in associa- 

 tion with D. hifidus and D. nanus. 



Several specimens are preserved in the collections of the British Museum 

 (Nat. Hist.), the Woodwardian Museum, and in the private collections of 

 Lapworth and the Authors. 



Nicholson's original specimens are not knoAvn with certainty, but we regard 

 that figured in PI. II, fig. 4 a, as our type. 



