MONOGRAPTUS. 365 



direction of tlie poljpary ; they overlap about one-half their length, and are about 

 twenty times as long as broad ; the apertural margins are even and approximately 

 horizontal. In the more distal portion of the polypary the thecfe are about the 

 same length, but are only ten times as long as broad, have a higher inclination, 

 and overlap for fully two-thirds of their length ; they expand somewhat in the 

 apertural region and as a i-esult the walls show a tendency to slight curvature ; 

 the apertural margins are even and everted. The thecae are so slender even 

 in the distal portion that a horizontal line drawn across the polypary at any 

 point would cut three different tliecaj at least. 



Remarks. — ^Tornquist, in his original description of this species, stated that he 

 was unacquainted with the distal extremity of the polypary. The British form 

 here described agrees so closely with Tornquist's species as regards the proximal 

 end that it is practically certain that the two are identical ; the features of the 

 tliecce of the distal portion of the polypary are, however, so striking and distinct 

 from those of the proximal portion that the British specimens were described 

 by Jones as a new species — Monog. rJieidoIensis. 



Affinities. — In the length of the thecse, their slender nature and amount of 

 overlap, Monog. dcinaces approaches M. leptothcca, with which it seems to have 

 been formerly confused ; the distal tliecEe of M. leptotheca , however, show no such 

 characteristic expansion in their apei'tural regions, and are even more slender than 

 those of M. acinaces. 



Horizon and Localities. — Llandovery (Birkhill Shales), zone of Af. cyplms. 



G. Wales: Eheidol Gorge. S. Scotland: Waterfall, Long Linn, Dobb's 

 Linn, etc. 



Associates, etc. — Monog. acinaces appears to be a fairly abundant fossil in the 

 zone of M. cgpJius, especially in the Pont Erwyd district, where Jones records it in 

 a band below that in which M. cijplius is most abundant. It has also been found 

 in S. Scotland in the zone of M. cyplms . 



Collections. — Jones, Lapworth, and Geological Survey of England and Wales. 



Monograptus gregarius, Lapworth. Plate XXXVI, figs. 3 a — d. 



1851. Graptotites Nilssoni, H;irkness, Quart. Journ. Geol. Soc, vol. vii, p. 61, pi. i, figs. 7 a — d. 



1888. Graptolites Nilssoni (pars), Nicholson, Quart. Jouru. G-eol. Soc, vol. xxiv, p. 537, pi. xx, fig. 19. 



1876. Monograptus gregarius, Lapworth, Geol. Mag., dec ii, vol. iii, p. 317, pi. x, figs. 12 a — c. 



1876. Monograptus gregarius, Lapworth, Cat. West. Scott. Foss., pi. 1, fig. 7. 



1877. Monograptus gregarius, Lapworth, Proe. Belfast Nat. Field Club, p. 131, pi. v, fig. 4. 

 1892. Monograptus gregarius, Toruquist, Siljausomr. Graptol., Acta Univ. Luud., vol. xxviii, p. 8. 

 1897. Pristiograptus gregarius, Freeh, Leth. Geogn., pt. i, vol. i, pi. i, figs. 3 — 5, p. 660, fig. 215. 

 1899. Monograptus gregarius, Tornquist, Monograptidse of Scaniau Bastrites Beds, Luuds Univ. 



Arssk., vol. XXXV, pt. 2, no. 1, p. 4, pi. i, figs. 1 — 6. 



