34 BRITISH GRAPTOLITES. 



Stipes about 5 cm. in length, widening- steadily from their origin to a maximum 



of 2'5 mm., diverging from a conspicuous sicula to form an inverted V, but 



expanding somewhat abruptly afterwards to include a more open angle. 



Thecee eight to ten in 10 mm., inclined to axis at angle increasing from 85° 



in the proximal part of the stipe to 55° in the distal, about four times as long 



as wide. Outer margins free one fifth their length. Apertm^al margin 



slightly concave, normal. 



Description. — The stipes widen gradually from their origin ; at first they are 



only '8 mm. wide, Ijut attain a maximum breadth of 2*5 mm. The curvature of 



the stipes is at first concave and then convex. The V shape is ver}^ ojjen and 



prolonged, the initial angle of divergence Ijeing about 90°, subsequently 40°, and 



finally about 130°, though this ultimate angle is very variable, as is also the 



proportion of the whole occupied by the V. 



The sicula is long and nari'ow, it is about 

 Figs. 2laand /j.— Di%»iO(/rf(j)<«s 2-5 mm. in length ; the first theca originates at a 



v-fractus, Salter. . [ 



point nearly midway between the aperture and the 

 apex, but rather nearer to the latter. It curves 

 away from the sicula at once, leaving it isolated at 

 its oral extremity for a considerable distance on 

 the left side (obverse view) ; it is short, and its 

 aperture is very slightly below the level of that 

 of tlie sicula. The crossing canal is very con- 

 „ . 7 1 /' 1 spiciious; it runs obliquely across the sicula from 



a. i'loxiuKil end iis a cast, obverse ^ i «/ 



view. Eiiiargeinent of part of the poiut of orisin of til. 1' to its apcrture. 



PI. II, fig. 10 a. i^, n, P 1 1 -, 



A. Proxiiiiai end, reverse view, siiowing The walls of the tliecas are always curved, 



crossiug canal in relief. Enlarge- ,ii -iii -^i i_ ii 



nient oFpart of PI. II, fig. 10?> though a gradual change m the curvature takes 



place in passing from the initial to the distal 

 regions of the stipes. The curvature of the earliest developed thecpe is concave, 

 but this changes gradually to a convex curvature, causing an alteration in the 

 direction of growth of tlie stipes, and with this change the angle of inclination 

 of the thecse rises. Near the sicula the theca3 are free for from two thirds to 

 one half of their length, l)ut only for one fifth of their length in the more distal 

 parts of the stipes. The average length of each mature theca is about 3 mm., the 

 average width '8 mm. 



Affinities. — This species may be readily distinguished from almost any other 

 by its peculiar form. The only one it resembles in this respect is IK (trfle.vK---, 

 which is a much more slender species, and smaller in every Avay. 



Horizon and Locality. — Arenig, Middle Skiddaw Slates (Dichograptus beds). 



LaJce District : B\ittermere. 



Associates, etc. — It occurs in abundance in certain parts of the Skiddaw Slates 

 associated with Tetraiiraptus fjvadribracliiafiis and large specimens of Temnograptiis 



