50 



BRITISH GRAPTOLITES. 







Groii}3 of D 



Murchisoni. 











Sub-group of D. bifidus. 



! 



Sub-group of D. indentus. 



D. Murchi- 

 SOili. 



Var. ffemi- 

 nix. 



D. bifidus. 



D. amplus. 



J / D. nanus. D. artus. D. stabilii-. 

 dentus. 



1 ' 1 



Form of stipe 



Parallel 



Divergent 



Divergent 



Divergent 



Parallel 



1 1 



Parallel Nearly Parallel 



parallel i 



Width . . . 



3 — 4 mm. ; 



2"5 mm. ; 



26 mm. ; 



6 4 mm. ; 



12 mm. 



1'3 mm. r3 mm. 16 mm. 





increase 

 slow 



increase 

 rapid 



increase 

 continuous 



increase 

 very rapid 











Leng-th of si- 

 cula 



3 mm. 



2 '6 mm. 



21— 2-4 

 mm. 



4 mm. 



2-5-3 

 mm. 



2-4 mm. 



13 mm. 25 mm. 



Origin of stipe 



Suboral 



Suboral 



Suboral 



Suboral 



Suboral 



Suboral Suboral Suboral : 



No. of thecae in 

 10 mm. 



14—12 



12 



15-13 



9—8 



10-8 



12—10 19—18 . 14 



Inclination 



45° 



45° 



45° 



35°— 40° 



25°— 30' 



30° 50° 40° 



Overlap . . 



1 4 



1 2 



2 3 



1 .•) 



2 4 



1 4 



2 5^ 



0-i 



i 2 1 1 X 



2 3 2 3 1 3 



Reflexed Sekles. 



Didymograpti in which the stipes are flexed and are directed upward and back- 

 Avard towards the nema, the initial angle of divergence being more than 180". 



Group VI. — Type D.fasciculatns. 



Didymograpti in which the stipes are at first convexly and then concavely 

 curved in an upward direction ; the thecse are very long and narrow, and are in 

 contact for the greater part of their length. 



Didymograptus fasciculatus, Nicholson. Plate II, figs 8 a — r. 



1869. Bidiimoyrapsus fascicAilai us, Nicholson, Ann. Mag. Nat. Hist. [4], p. 241, pi. xi, tigs. 21, 22. 



1870. Didymograpsus fasciculatus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 344, figs. 5 a, b. 

 1898. Didymograptus fasciculatus, Elles, Quart. Journ. Geol. Soc, vol. liv, ]■>. 507, fig. 24. 



Stipes with graceful concave curvature, from 2' 5 to 5 cm. or more in length, 

 very narrow at origin, but widening distally to maxinunn of 1*1 mm. Sicula 

 short and broad. Theca? nine to ten in 10 mm., of extraordituiry length 



