408 



BRITISH GRAPTOLITES. 



expansion in the apertnral region and hence no denticle or appearance of retro- 

 version of the apertnral margin. 



Horizon ami Localities. — Llandovery (zone of M. SechjioicJcii). 



Ireland : Limehill, Co. Tyrone. ;S'. Scotland : Dobb's Linn ; Belcraig Bnrn, etc. 

 Lnlr District: Long Sleddale ; Skelgill. Wales: Pary's Monntain, Anglesea; 

 Quarry near River Twymyn, Llanbrynmair ; B.S.E. Fuches Gau Farm House, 

 Pont Erwyd, etc. 



Associates, etc. — Monog. tenuis occurs in some abundance in the M. Sedgwicldi 

 zone, in which it often characterises a distinct band or sub-zone (Lake District 

 and C. Wales). It is commonly associated with Monog. Sedgivickii, M. limatnlus, 

 M. argenteus var. cygneus, Ghjptog. serratns, and Climacog. scalaris, etc. 



Collections. — Geological Survey of Scotland, Geological Survey of England 

 and Wales, British Museum (Natural History), Sedgwick Museum, Greenly, Jones, 

 Lapworth, and the Authors. 



Monograptus argutus, Lapworth. Plate XL, figs. 3 a—e. 



1876. Monograptus argutus, Lapworth, G-eol. Mag., dec. ii, vol. iii, p. 318, pi. x, figs. 13 a — c. 



Figs. 274 a—f. — Monograptus argutus, Lapworth. 



Polypary several cm. in length, arcuate distally, and involute proximally, 



widening steadily to a maximum 

 breadth of 1"5 mm. Thecte eight 

 in 10 mm., with conspicuous sig- 

 moid curvature of their walls, 

 overlapping fully one-half their 

 length, and with introverted and 

 slightly introtorted apertnral 

 margins opening within pouch- 

 shaped excavations. 

 Description. — The polypary is slender 

 and several cm. in length ; it is fairly 

 robust in the initial portion, and the 

 increase in width takes place wholly 

 within the first few cm. of length, so 

 that distal fras^ments have a uniform 

 breadth. 



The thecj© do not exceed 3 mm. in 

 length; in their general characters — 

 namely, the pronounced sigmoid curva- 

 ture of their walls and the introversion and introtorsion of the apertnral margin— 



a h c d e f 



a. Proximal portion, showing dorsal curvature. Enlarge- 

 ment of part of PL XL, fig. 3 h. 



h. Portion near proximal end ; preserved mainly as a 

 cast. Skelgill ; Stockdale Shales. Coll. Lapworth. 



c and d. Portions of same specimen, showing the different 

 appearances presented by the thecse. Parts of speci- 

 men on same slab as fig. 274 h. 



e. Adult theciE ; preserved partly as a cast, partly in 

 relief. Enlargement of part of PI. XL, fig. 3e. 



/. Adult theca', compressed. Another part of specimen 

 fig. 274 e. 



