10 BRITISH GRAPTOLITES. 



Didymograptus nitidus (Hall). Plate I, figs. 2 (/ — c. 



1858. Graptolithus nitidus, Hall, Eep. Geol. Survey Canada, 1857, p. 129. 



1865. Graptolithus nitidus, Hall, " Grapt. of Quebec Group," Geol. Surv. Canada, Canadian Organic 



Kemains, dec. 2, p. 69, pi. i, figs. 1 — 9. 

 1868. Bidymograpsus nitidus, Nicholson (pars), Quart. Joui-u. Geol. See, vol. xxiv, p. 135. 

 187U. Non Bidymograpsus nitidus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 341, figs. 3 



a — c. 

 1874. ? Didymograptus nitidus, Etheridge, jun., Ann. Mag. Nat. Hist. [4], vol. xiv, p. 6, pi. iii, 



fig. 20. 

 1898. Didymograptus nitidus, Elles (pars). Quart. Jouru. Geol. Soc, vol. liv, p. 499, fig. 20. 



Stipes 2 — 5 cm. in length, narrow at origin, bnt increasing in width throughont 

 up to 1'6 mm., diverging from a small sicnla at wide but varying angles, 

 and growing subsequently in an approximately horizontal direction. Thecas 

 twelve to thirteen in 10 mm., inclined at a large angle, three times as long- 

 as wide, free one third their length. Apertural margins normal, slightly 

 concave. 

 Description. — The stipes measure at their origin about "87 mm. in width, but 

 widen gradually up to a maximum of 2*1 mm. ; this width is only attained in the 

 largest specimens, those more commonly met with do not exceed 1"6 mm. ; there is 

 always diminution at the distal extremities, owing to the incomplete development 

 of the thecas. The greatest length observed in any one stipe is 5" 7 cm. In some 

 specimens the stipes diverge from each other at a primary angle of 175^, and their 

 dorsal walls are straight ; but in others there seems to be a tendency to grow at first 

 downward, the angle of divergence being about 130°, In exceptional cases this 

 downward growth may be continued, but more frequently it is arrested after the 

 development of six to ten thecse on each stipe, and then the horizontal direction is 

 gradually assumed : in this case the dorsal walls are at first curved, but subse- 

 quently straight. In other cases again, where the downward growth is arrested, 

 the stipes are directed slightly but persistently upward, and the dorsal walls are 

 slightly but persistently curved. 



The sicnla is small, it measures commonly about 

 FiG.5a.-Didi/moj(rajptwsnaidKs(Haii). i-<^ ^y^^ {y^ length; it is decidedly broad in propor- 



^^>£jV tion to its length, and tapers abruptly towards the 



[a >S> apex. It is often placed somewhat obliquely with 



Proximal end preserved as a cast, ob- regard to the stipcs. Til. 1^ ariscs from the sicula 



verse view. Randal Crag, Skiddaw. i t ^ i i • t 



Cou. Woodwardian Museum. at a sliort distaucc Delow its apex, and grows down 



some little way before originating the crossing canal, 

 which traverses the sicula approximately horizontally. It should be noticed that 

 th. 1^ is closely appressed to the sicula till the aperture is reached, at which point 

 it curves gradually away. 



The thecae are 1'6 — 2 mm. in length ; they are narroAV at their bases, but wider 



