208 BRITISH GRAPTOLITES. 



The thecae are highly characteristic (see Fig. 116), exhibiting- a well-marked 

 .sigmoid curvature which is most sudden in its central part, and having their 

 apertural margin slightly introverted. They grow at first upward, then bend 

 outward almost at right angles, and finally turn upward at right angles, appearing to 

 undergo a certain amount of torsion in the process. They are markedly alternate. 

 The apertural margins are situated in wide, deep excavations which occupy about 

 one-third the width of the polypary in the distal region, but rather less proximally. 



The virgula may be distally prolonged for a distance of from 2 to 3 cm. When 

 compressed the characteristic zig-zag septal groove is often indiscernible. In 

 the scalariform view the thecae present few of their characteristic features; there 

 remain, therefore, only the general outline with its sub-parallel sides and the 

 character of the proximal end for purposes of identification. 



Affinities. — The zig-zag septal groove is highly characteristic; it is found in two 

 other species, Gl. Hughesi and Gl. extremus, but these are both more diminutive 

 species in every way. The shape of the proximal end and the characters of the 

 thecal are sufficient to distinguish the form when well preserved. 



Horizon and Localities. — Upper Arenig (zone of Didymog. bifidus) to Lower 

 Hartfell (zone of Gl. Wilsoni). 



8. Scotland: Dobb's Linn; Hartfell; Glenkiln Burn; Craigmichan Scaurs; 

 Black Linn; Balclatchie Bridge; Laggan Gill; Penwhapple Glen, etc. Wales: 

 Tiddyndicwm ; Pont Seiont ; Llanvirn ; Porth Hayog, Ramsey Island. Lake 

 District: Thornship Beck. Ireland: Ballygrot; Craigavad ; Coalpit Bay. 



Associates, etc. — Gl. Scharenbergi has a fairly long range in time, from the 

 Upper Arenig to the Lower Hartfell, but it is very rare in rocks of Upper Arenig 

 age. It seems to attain its' maximum in individuals in the Upper Glenkiln Shales 

 and Lowest Hartfell Shales of S. Scotland. 



1 In the Upper Arenig rocks of Pont Seiont and Porth Hayog it occurs asso- 

 ciated with Didymog. bifidus and Gl. (?) confertus ; in the Glenkiln Shales its com- 

 panions are Nemog. gracilis, Dicellog. serious, and Gl. bicomis, and in the Lowest 

 Hartfells it occurs with Gl. Wilsoni, Gl. bicomis, and Dicranog. Nicholsoni. 



Collections. — Sedgwick Museum, Lapworth, and the Authors. 



Climacograptus Hughesi (Nicholson). Plate XXVII, figs. 11 a — e. 



1853. Dijilotjnijisux frreiiusculus, Richter, Zeitschr. d. deutsch. geol. Gesell., vol. v, p. 456, pi. xii, 



figs. 11—13. 

 1869. JDiplograpsus Hughesi, Nicholson, Ann. Mag. Nat. Hist., ser. 4, vol. iv, p. 234. 

 1871. Diplograptus teretiusculus, Richter, Zeitschr. d. deutsch. geol. Gesell., vol. xxiii.pl. v, tigs. 5 — 7. 

 1876. Non Diplograptus Hughesi, Lapworth, Cat. West Scot. Foss., pi. ii, tig. 37. 

 1882. Climacograptus undulatiis, Kurck, Geol. Foren. i. Stockh. Furhandl., vol. vi, p. 303, pi. xiv, 



fig. 11. 

 1890. Climacograptus inter nexus, Tornquist, Siljansomr. Graptol. I, Acta Univ. Lund., vol. xxiv, 



p. 25, v \. ii, figs. 8, 9. 



