122 BRITISH GRAPTOLITES. 



'5 mm. to 1 mm. in the different examples, so that some are relatively wide when 

 compared with others. 



The secondary branches agree with the main stipes in all essential characters ; 

 they usually originate in the early stages of growth of the main stipes, being absent 

 at their more distal extremities, but this is not invariably the case ; they are 

 typically disposed in pairs, but there is considerable diversity in the arrangement 

 of these pairs, and in some cases one member of the pair appears to be suppressed 

 (A. divergens, var. radiatus). 



As in Leptograptus, a central stipe, or branch, is frequently developed from the 

 sicula or its immediate neighbourhood. 



The thecas are rarely shown compressed in true profile, their apertures either 

 facing, or being opposed to the view of the observer ; this condition of preserva- 

 tion seems to be due to the radial disposition of the branches. 



The genus Arnphigraptus is somewhat rare in British deposits, and has only 

 been found in the Hartfell Shales in the zones of Dicranog. clingani and Pleurog. 

 linearis, and it is exceedingly rare in the first-named zone. 



The following forms may be recognised as belonging to the genus : 



A. divergens ; A. divergens, var. radiatus ; A. distans. 



Arnphigraptus divergens (Hall). Plate XVIII, fig. 1. 



1859. Graptolithus divergens, Pal. New York, vol. iii, Suppl., p. 509, fig. 9. 

 1865. Graptolithus divergens, Hall, Grapt. of Quebec Group, pp. 12, 13, fig. 11. 

 1876. Arnphigraptus divergens, Lapworth, Cat. West. Scott. Foss., pi. iii, fig. 70. 



Main stipes narrow, 11 cm. or more in length, straight and somewhat rigid, or 



very slightly curved throughout, diverging at about 180° from the sicula, 



which is rarely distinguishable. Secondary branches, simple or compound, 



5 — 6 cm. or more in length, also somewhat rigid or with very slight curva- 



ture ; apparently arranged in pairs, usually three in number, on either side of 



the sicula and in close proximity to it. Thecse long and narrow, eight to 



nine in 10 mm., inclined at 10°, about eight times as long as wide, free for 



two thirds their length. Apertural margins introverted, occupying about 



one third the total width of the stipe. 



Description. — The main stipes and secondary branches appear to be of 



approximately uniform breadth for the greater part 



«erge«,s(HaU). of fcJie i r i en g t h ; they are about -8 mm. wide, except 



at their point of origin where they are rather less. 



The secondary branches are very occasionally com- 



lii iii fchecae. Enlargement "I' part of 



specimen on same slab as PI. XVIII, ljouild 

 fig. 1. 



The sicula appears to be '7 mm. in length. 



