"Vol. 54.] i^AUlTA OF THE SKIBDAW SLATES. 507 



connecting-canal is very oblique. The stipes diverge at an angle 

 of 180°. 



The earliest theca arises from the middle of the sicula and grows 

 straight out, so that the stipe to which it gives rise appears to 

 originate at that point ; the theca (th. 1") fiom which the secondary 

 stipe originates does not diverge from the sicula till close to the 

 aperture, so that the secondary stipe appears to arise close to the 

 aperture of the sicula. 



The dorsal wall of each stipe shows a slight convex curvature. 

 There are eighteen thecse in the space of an inch (seven in 10 mm.). 



I am not aware that Didymogi^a^tus gracilis has been recorded 

 hitherto from this country. 



Occurrence. — Middle Skiddaw Slates. 



Locality. — Barf, near Keswick. 



{d) DiDYMOGPvAPTTJS PASCICtTLATUS, Nich. 



1869. Didymog7'apsus fasciculatus, Nich. Ann. Mag. Nat. Hist. ser. 4, vol. iv, 

 p. 241 & pi. xi, figs. 21 & 22. 



1870. I)idymogra]psus fasciculatus, Nich. ibid. vol. v, p. 344, figs, oa & b. 



This species should be easily recognized, even in a fragmentary 

 condition, because of its peculiar characters. All the specimens 

 hitherto recorded belong to the Skiddaw Slates. None are perfect, 

 showing at most the proximal end and one stipe. The two 

 specimens in the Woodwardian Museum, which I believe to be 

 referable to this species, are fragments of the distal parts of the 

 stipes. 



The dorsal wall of the stipe seems to be always convexly curved, 

 but not always equally so. The stipe at first grows straight up- 

 wards, and then curves round ; in some specimens the curvature is 

 continued to such an extent that the distal portion of the stipe is 

 parallel to the proximal part, but in others, after the first decided 

 convex curve, the stipe runs very nearly straight. 



The cells are peculiar and eminently characteristic ; they number 

 twenty-four in the space of an inch 



(nine to ten in 10 mm) in the adult pig, 24.-Tivo specimens of 

 part of the stipe, and are exceed- Didymograptus fasciculatus 

 ingly long and narrow, attaining sent hy Prof. H.A.Nicholson, 

 sometimes a length of nearly ^ inch ^^.^^^^ Ellerqill. 

 (6-3 mm.). They are inclined at so 

 low an angle (15°) tbat they seem to 

 run nearly parallel to the back of 

 the stipe ; they are free only for 

 the last fraction of their length, 

 and have their apertures perpen- 

 dicular to the axis of the stipe. In l. 2. 

 the proximal part of the rhabdo- [Nat. size.] 

 soma the cells are smaller, more 



distant, and are in contact with each other only for a small fraction 

 of their length. 



A perpendicular line dropped near the aperture of a cell will be 



