DIDYMOGRAPTUS. 



35 



'laalHpJex, Troclwgraptus diffusus, and Hologroptus Deanci. The best specimens, 

 Avliicli are excellently preserved, are in Lapwortli's collection. We have not seen 

 Salter's type specimen, but Ave regard the one figured in our PI. II, fig. 10 a, as 

 the typical example for this species. 



Fig. 22. — Did7jmnrp-aptus v-fractus, 

 var. volucer, H. O. Kicliolson. 



Didymograptus v-fractus, var. volucer, H. 0. Nicholson. Plate II, figs. 11 a, h. 



1890. Didymograptus v-fmcfiis, var. volucer, H. O. Nicholson, Geol. Mag. [3], vol. vii, p. 342, fig. 3. 



Description.- — In addition to the typical form of the species v-fractus there 

 occurs in the same beds a form which exhibits certain differences from the type, 

 and as these differences seem to be constant it has been separated by Dr. H. 0. 

 Nicholson as a distinct variety. 



In this variety the basal angle is narrower than 

 in the typical form, and the stipes at the termination 

 of the V are bent abruptly at right angles to their 

 original direction of growth, and continue to grow 

 thereafter in a horizontal direction. The maximum 

 Avidth, 2"1 mm., is attained at the bend. The angle 

 of inclination of the thecas in the V part is 25° — 30°, 

 at the l)end 00°, and in the horizontal part 40°. 



Horkoii (iiid Locdliti/. — Arenig, Middle Skiddaw 

 Slates. 



Lal-p District : Outerside. 



The type specimen is now in the collection of 

 Professor Lapworth. Other specimens are in the 

 Woodwardian Museum and in the Museum of the Geological Survey. Var. volucer 

 lias not lieen found associated with any other forms. 



Proximal end, reverse view. Enlarge 

 uient of part of Pi. II, fig. Uh. 



Didymograptus deflexus, sp. nov. Plate II, figs. 12 a, c. 



1898. Didymoijrajitus v-fractus, Elles {pars). Quart. Journ. Geol. Soc, vol. liv, p. 508, figs. 25 — 28. 



Stipes 2"5 — 5 cm. in length at origin, narroAV, widening fairly rapidly to a 

 breadth 1 mm., diverging from a conspicuous sicula to form an inverted V, 

 but expanding afterAvards to include a more open angle. Thecee fourteen in 

 10 mm., inclined to the axis at angles 25° — 30°, about three times as long 

 as wide, outer margins free one half to two thirds their length ; apertural 

 margin normal, slightly concaA'e. 



Description. — The stipes are narroAvest at their origin, but they expand fairly 



