Vol. 54.] FAUNA OF THE SKIDD AW SLATES. 489 



1865. Graptolithus Bigshyi, Hall, 'Grapt. of the Quebec Group,' Geol. Surv. 

 Canada, dec. 2, p. 86 & pi. xvi, figs. 22-30. 



The specimens upon which Salter founded his species Didymo- 

 graptus caduceus seem to have belonged to two distinct forms, 

 D, gibherulus, Nich., and the Tetragraptus to which Hall later gave 

 the name Bigshyi. To avoid any confusion which might therefore 

 arise, it appears wisest to adopt Hall's specific name in place of the 

 older term. 



This species is abundant in the Skiddaw Slates ; it is characteristic- 

 ally small and never seems to exceed -J inch (19 mm.) in length, 

 while specimens of less than | inch (12-7 mm.) are more commonly 

 found. The general form of the rhabdosoma may be described as 

 a broad oval, truncated at its upper end in most specimens, but 

 occasionally nearly complete, since there is a tendency on the part 

 of some stipes to come together at their distal extremities, and in 

 some cases actual fusion appears to have taken place. 



The stipes are about ^ inch (1-05 mm.) wide at their origin, but 

 widen rapidly to a maximum width of J inch (3-2 mm.), diminishing 

 again at their distal ends. The stipes are distinctly broad in pro- 

 portion to their length ; their dorsal wall is nearly always concavely 

 curved, though the amount of curvature varies greatly in different 

 individuals. 



The sicula is apparently about Jj i^^^ (^"1 nim.) in length, and 

 the proximal part of the rhabdosoma bears a general resemblance 

 to Didymograptus gihheridus, Nich. Holm's work on the structure 

 of this species indicates that the earliest theca arises from the sicula 

 near its apex, and the two stipes of the 1st order, constituting the 

 * funicle ' of other authors, are reduced to a minimum, for dicho- 

 tomous division appears to take place at once.^ 



The thecse number between thirty-two and thirty-six to the inch 

 (thirteen to fourteen in 10 mm.). They are at first nearly at right 

 angles to the general direction of the stipe ; those developed later 

 have an initial angle of 40° to 50°, but curve so that the inclination 

 near the aperture makes an angle of 60° to 70° with the axis. The 

 thecae are widest at their apertures, and are about 4 times as long 

 as wide. The apertures are slightly concave and mucronate, and 

 the apertural angle is about 140°. 



Occurrence. — In the lowest part of the Middle Skiddaw Slates, 

 with Bryograptus ramosus var. cumhrensis. 



Localities. — Randal Crag, Gibraltar; White House Pell, Skid- 

 daw ; Bassenthwaite Sand-beds ; Outerside ; Troutbeck. 



Note. — 1 have not been able to convince myself that the form 

 provisionally named inosculans by Marr & Nicholson (Geol. Mag. 

 1895, p. 538). is really distinct from the species above described. 

 There seems to be every intermediate stage between a form in which 

 the stipes are distinctly separated and that in which complete 

 fusion appears to have taken place, and the characters of the cells 

 are in every case the same. In my opinion, the real intermediate 

 form between T. Bigshyi (Hall) and PJiyllograptus is to be found 



^ Holm, 'Om Didymograptus, Tetragraptus, och Phyllograptus,* Geol. Foren. 

 Stockholm Forhandl. vol. xvii (1895) p. 319. 



