NEMAGBAPTUS. 127 



Nemagraptus gracilis (Hall). Plato XIX, figs. la—/. 



1847. Graptolitlms gracilis, Hall, Pal. New York, vol. i, p. 274, pi. lxxiv, figs. 6 a, b, c, d. 



1851. Bastrites Barrandi, Harkness, Quart. Journ. Geol. Soc, vol. xi, p. 475. 



1855. Nemagrapsus elegans, Emmons, American Geology, vol. i, p. 109, pi. i, fig. C. 



1859. Graptolithus gracilis, Hall, Pal. New York, vol. iii, Sirppl., p. 510, fig. 5. 



1862. ? Graptolithus gracilis, Baily, Quart. Journ. Geol. Soc. (Dublin), vol. ix, p. 5, pi. iv, fig. 5. 



1805. Graptolithus gracilis, Hall, Grapt. of Quebec Group, p. 13, fig. 10 ; p. 14, fig 17. 



1806. Stephanograptus gracilis, Geinitz, Neues Jahrb. f. Min., p. 124. 



1867. ? Graptolithus gracilis, Baily, Mem. Geol. Survey Ireland, Sheet 133, p. 12, fig. 3. 



1868. Cladograpsns gracilis, Carruthers, Geol. Mag., vol v, p. 130. 



1808. Ccenograptus gracilis, Hall, 20th Ann. Rep., p. 217, figs. 17 (?), 18, 19. 



1868. Helicograpsus gracilis, Nicholson, Ann. Mag. Nat. Hist. [4], vol. ii, p. 25, fig. 1. 



1876. Ccenograptus gracilis, Lapworth, Cat. West. Scott. Foss., pi. iii, fig. 65. 



1877. Ccenograptus gracilis, Lapworth, Proc. Belfast Nat. Field Club, p. 142, pi. vii, fig. 11. 



Main stipes from 2 — i'h cm. in length, but never exceeding "6 mm. in width, 



diverging from a conspicuous sicnla at 180°, but immediately after showing 



marked curvature in opposite directions, forming a broad S-shaped bend. 



From the outer side of each curve secondary branches are given off at 



regular intervals, commencing close to the sicnla, then curving in same 



direction as main stipe from which they arise. Thecal seven to nine in 



10 mm., inclined 10° — 15°, about 2 mm. in length, six times as long as wide, 



free for two-thirds to three-quarters their length. Apertural margins 



concave, slightly introverted. 



Description. — The main stipes of the polypary are most conspicuously curved. 



The secondary branches are often of great length, and curve in the same direction as 



their main stipe, but to a smaller extent. These secondary branches increase in 



width from ' i mm. to '9 mm.; they vary very much in length and in number, 



according to the age of the individual. Fully developed forms may have as many as 



fourteen secondary branches on each main stipe, while in young individuals the 



number may not exceed three or four. The earliest secondary brandies arc always 



developed close to the sicnla, the first at a distance of about 2*5 mm. from the 



sicnla, and the rest 1"5 — 2 mm. apart; each appears to arise opposite the aperture 



of a theca. 



The sicnla is small (1 mm.), but owing to its central position and to the slender 

 nature of the polypary it is always conspicuous. 



Fn"i. 7<> ". — Nemtt<)?in>tns i/rftriUs (Hall). rin a l l\ ' • ■ j. • 1 



Ihe first theca seems to originate at a point midway 

 between the apex and the aperture of the sicnla, or 

 perhaps very slightly nearer to its aperture; the 

 crossing canal is horizontal, and th.l 1 and th.l 2 are 

 Proximal end. ^niargement of part of of approximately equal length, so that the whole 



17 



