24 BEITISH GRAPTOLITES. 



Affinities. — The species is closely allied in form to /-'. NichoJsoni, from wliich it 

 may, however, be distinguished — 



(1) by the greater tenuity of the stipe, 



(2) the characters of the thecas : 



{(') low angle of inclination, 



(//) short contact, 



[c) number in a given unit of length, 



(3) the character of the proximal end. 



Horizon and Localities. — Arenig (Middle and Upper Skiddaw Slates) ; Upper 

 Arenig (Shelve and St. David's). 



Lake District : Aik Beck, Pooley ; Barf ; Carlside Edge, Skiddaw ; Ellergill. 

 *S\ Shropshire : Shelve; Hope Valley. St. David's District: Llanvirn Quarry; 

 Porth-hayog, Ramsey Island. 



Associates, etc. — The species occurs in association with D. nanus, and is fairly 

 abundant at certain of the above localities. Good specimens are in the collections 

 of the Geological Survej^ the Woodwardian Museum, the British ]\Iuseum 

 (Natural History), and in that of Professor Lapworth. 



The original specimen figured by Nicholson is unknown, but that illustrated on 

 our PI. II, fig. 1 />, now in the British Museum of Natural History, bears the 

 label D. affinis in Nicholson's handwriting, and may be regarded as the type. 



Didymograptus gracilis, Tornquist. Plate II, fig. 2. 



1890. Didijiuocjmjjtns gracilis, Tornquist, "Uudersok. ofver siljansomrSdets. Graj^t., i)t. 1." Lunds 



o 



Univ. Arssk., vol. xxvi, p. 17, pi. i, figs. 9 — 12. 

 1898. Didymograptus gracilis, Elles, Quart. Joui-n. Geol. Soc, vol. liv, i>. 506. 



Stipes slightly curved, aljout o cm. in length, very slender, never exceeding 



•5 mm. in width, diverging at 180° from a conspicuous sicula at markedly 



different levels. Thecte widening towards these apertures, seven in 10 mm., 



free throughout their length, inclined at 15° or rather less, outer walls 



slightly curved. Apertural margin straight, perpendicular. 



Description. — The narrow stipes are of approximately uniform width, and the 



greatest Ijreadth, which never exceeds -5 mm., is attained opposite the apertures 



of the thecge. The stipes may attain a length of 5 cm. 



The sicula is more than ordinarily conspicuous because of the slender character 

 of the polypary. It measures about 1-2 nnn. in length. The apparent origin of 

 tlie stipes at different levels on the sicula must be due to the very oblique course 

 taken by the crossing canal, though this has not been directly observed, the only 

 specimen sufficiently well preserved to show such detail being presented in the 

 obverse view. Theca V originates two thirds of the way down the sicula and 



