DICELLOGRAPTUS. 153 



Horizon and Localities. — Hartfell Shales. (Zones 



Figs. 95 b, c, and d. — Dicellograptus , ... ... 



Forchammeri, var flexuosus, Lapw. of ( 'It IIIH COlf . WllsOTll and I >icr<l iliiij . ( 'it m/it It /). 



Proximal thecae, showing mesial 



$. Scotland: Hartfell; Glenkiln Burn; Syart 

 Law ; Garryhorn Burn ; Rein Gill. 



Associates, etc. — Var. fiexuosus occurs in the zone 

 of Glimacog. Wilsoni associated with Climacog. 

 Scharenbergi and the zone fossil. It is, however, 

 of commoner occurrence in the zone of Dicranog. 

 Clingani, where it occurs in company with Leptog- 

 T n i!i TT s P ecimen on same slab as flaccidus, Diploq. truncatus, and their usual associates. 



PL XXII, fig. 2 b. J . 



More distal thecae. ibid. The best specimens known are in Lapworth's col- 



Sralariform view. Specimen on same 



slab as pi. xxii, figs. 2 a and 2 a. lection and in that of the Geological Survey of 

 Scotland. 



Dicellograptus sextans, Hall. Plate XXI, figs. 1 a—e. 



1843. Graptolitluis sextans, Hall, Pal. New York, vol. i, p. 273, pi. 74, figs. 3 a — e. 



1849. Or aptolithus sextans, Salter, Quart. Journ. G-eol. Soc, vol. v, p. 17, pi. i, fig. 10. 



1855. Diplograptus? sextans, McCoy, Brit. Pal. Foss., p. 9. 



1865. Dicranograptus sextans, Hall, Grapt. Quebec Group, p. 57. 



1870. Didymograpsus sextans, Nicholson, Ann. and Mag. Nat. Hist. [4], vol. v, p. 356, fig. 9. 



1870. Dicranograptus sextans, Hopkinson, Geol. Mag., vol. vii, p. 356, pi. xvi, figs. 1 a — c. 



1870. Dicranograptus formosus, Hopkinson, Geol. Mag., vol. vii, p. 356, pi. xvi, fig. 2. 



1876. Dicellograptus sextans, Lapworth, Cat. West. Scott. Foss., pi. iii, fig. 78. 



1877. Dicellograptus sextans, Lapworth, Grapt. Co. Down, pi. vii, fig. 4. 



Stipes straight or with slight curvature, commonly 1 — 2 cm. in length, main- 

 taining an approximately uniform width of rather less than "8 mm. for 

 the greater part of their extent, though somewhat narrower proximally ; 

 diverging at 300° from an inconspicuous sicula ; virgella and lateral spines 

 conspicuous; axil pointed. Thecae short, thirteen to eleven in 10 mm., 

 overlapping about one fourth of their length ; free part of outer wall oblique, 

 curved. Apertural portion slightly introverted and introtorted, opening 

 within a pouch-shaped excavation, which occupies one third the width of 

 the stipe. 

 Description. — The stipes in the shorter forms are usually straight, but some 



of the longer specimens show slight concave curvature of their ventral margins. 



The angle of divergence is very constant and characteristic, as is also the pointed 



shape of the axil. 



The sicula is short and blunt, measuring only - 7 nun. in length. It never 



appears conspicuously within the axil, though its apex can occasionally be 



