30 



BRITISH GRAPTOLITES. 



Pigs. 18 a and h. — Didymograptus 

 serratulus, Hall. 



The number of tliecae varies in different parts of the stipes, being seven in 

 10 mm. near the sicula, but usually eiglit in 10 mm. in the more distal portions. 



The thecse are free for half their length in the distal 

 regions of the stipes, but for a greater fraction of 

 their length near the sicula. 



Bemarls. — D. serratulus is found in the Glenkiln 

 Shales of South Scotland and Ireland, but generally 

 in a fragmentary condition. It occurs usually as 

 isolated stipes, which agree, however, in all par- 

 ticulars with the American examples described by 

 Hall, except that the general form is rather more 

 curved than Hall's figures would seem to indicate is 

 the case with the American forms. 



Affinities. — D. sn'ratulns is closely allied to D. 

 NIchohovi and D. euodus, from both of which it 

 differs — 



(1) in tlie number of theca? in a given unit of 

 length, 



(2) in the greater inclination of the thecae. 

 Horizon and Localities. — Llandeilo (Glenkiln 



Shales). 



8. Scotland : Berrybush, Selkirksliire ; Birnock, Lanarkshire ; Minnoch AVater, 

 Glencaird Lodge. N. Ireland: Ballygrot, Craigavad (co. Down). 



Associates, etc. — D. serratulus occurs in abundance in association ^Wtli D. super- 

 stes, Cainograptus gracilis, Leptograptus, sp., Dicellograptus Moffateusis, Grijpto- 

 graptus tricornis, Glathrograptus cuneiformis, Lasiograptus himucronatus, and 

 Diplograptnis eughjphfis in the Glenkiln Shales of S. Scotland, where it has 

 been collected by Lapworth, and by the officers of H.M. Geological Survey of 

 Scotland. Lapworth also records the species from Ireland (co. Down). 



a. Proximal end shuwing long nerna, 

 reverse view. Enlargenient of 

 part of PI. II, fig. 7 a. 



h. Distal thecEe. Enlargement of part 

 of PI. II. fig. 7 6. 



Didymograptus simulans, sp. nov. Plate II, figs. 6 a, h. 



1868. Didijniograpsus Hitidus, Nicholson (pars), Quart. Journ. Geol. Soc, vol. xxiv, p. 135. 

 1870. Didymograpsus nitidus, Nicholson, Ann. Mag. Nat. Hist. [4], vol. v, p. 342, fig. 3. 

 1898. Didymograptus nitidus, Elles (pars). Quart. Journ. Geol. Soc, vol. liv, ]). 500, fig. 19. 



Stipes 2 — 5 cm. in length, curved, narrow at origin, but widening steadil}' 

 throughout their length to a maximum of 1"4 mm., diverging from a 

 slender sicula at about 115°, and including ultimately an angle of about 

 160°. Thecae thirteen to fourteen in 10 nun., inclined at 30°, three 

 times as long as wide, free half their length. Apertural margins 

 normal, straight, submucronate. 



