MONOGRAPTUS. 409 



they sliow a decided approach to the type regarded as characteristic of the genus 

 Bicellograftus. 



Affinities. — In the form of the polypary Monog. argutus comes, perhaps, nearest 

 to M. revolutns, but the characters of the thecse are distinct and, indeed, unique 

 amongst the Monograpti. 



Horizon and Localities. — Llandovery (Birkhill Shales), top of zone of M. gregarias. 



S. Scotland : Dobb's Linn. Wales : Old Quarry 270 yards N.E. Gwen-ffrwd 

 Uchaf Farm House, Pont Brwyd, etc. LaJce District : Skelgill. 



Associates, etc. — Monog. argutus is a fairly abundant fossil in the Upper Birkhill 

 Shales, immediately below the zone of M. convohitus, at a horizon corresponding 

 with the M. argenteus band of the Lake District and the M. leptotlieca band of the 

 Pont Erwyd locality. It occurs associated with Monog. leptotheca, M. Clingani, 

 M. communis, and other forms. 



Collections. — Sedgwick Museum, Geological Survey of England and Wales, 

 Jones, Lap worth, and the Authors. 



Group III. 



Monograp)ti in which the mature tlieca? are short tubes with abrupt sigmoid 

 curvature and some torsion of the thecal axis, so that the apertural margins show 

 occasionally within distinct excavations in the ventral margins. 



The thecae in this group show an approach in character to those of the M. 

 priodon type, but the apertural region is adnate instead of isolate and the torsion 

 of the thecal axis is but slight. There is often an appearance of narrowing 

 towards the apertural margin, but this is largely, if not entirely, induced by the 

 torsion of the thecal axis. As a result of this torsion the polypary viewed in the one 

 aspect shows thecee resembling those of M. diihius, while in the other they resemble 

 those of Climacograptus. The polyparies are approximately straight or slightly 

 flexed, . ^ 



Monograptus vomerinus (Nicholson). Plate XLI, figs. 1 a^e. 



1850. Graptolithus colonus, Barrande, Grapt. de Boheme, pi. ii, fig. 4. 



1851. Groptolithus colonus, Suess, Bulimisclie Graptolitlien, pi. viii, figs. a—f. 



1868. 6 raptoliies priodon, Nicholson, Quart. Jouru. Geol. Soc, vol. xxiv, pi. xx, figs. 7, 8. 



1872. Grai)toiites vomerinus, Nicholson, Monog. Brit. Grapt., p. 53, fig. 21. 



1876. Mo7iogra/ptus vomerinus, Lapworth, Geol. Mag., dec. ii, vol. iii, p. 353, pi. xii, figs. 6 a — e. 



1900. Monographis vomerinus, var. a, EUes, Quart. Journ. Geol. Soc, vol. Ivi, p. 403. 



Polypary 1 — 8 cm. in length, approximately straight, or slightly flexed at the 

 proximal extremity, widening rapidly from an initial breadth of "4 mm. to 

 a maximum of about 2 mm. within a short distance of the proximal end. 



