MONOGRAPTUS. 



I- 1 : 



Fins. 305 c — e. — Moinniriiphis Halli 

 (Barrande). 



taken effect at oblique angles to the plane above-mentioned. In proportion as 

 the degree of obliquity of compression increases, (1) the line marking the extreme 



visible edge of the aperture takes on a more 

 and more curved form; (2) the spine itself 

 becomes more and more foreshortened, and 

 correspondingly less and less conspicuous; (3) 

 simultaneously presents the appearance of 

 originating from a different point along the 

 apertural margin ; and (4) of proceeding in a 

 different direction. These different appear- 

 ances naturally answer to the varying perspec- 

 tive views of a specimen preserved in full relief. 

 Affinities. — From what has been said above 

 it is clear that Monog. Halli must be carefully 

 distinguished from M. Sed'jwickii from which it 

 differs in (1) general form, being straighter 

 and stiffer throughout, and (2) having distal 

 theca3 which have a greater amount of overlap, 

 show even less retroversion of the apertural 

 region, and have shorter and shorter apertural 

 spines. In certain specimens from the Pont 

 Erwyd district which are preserved in relief, the 

 spines are not at all a conspicuous feature and 

 the Tetroverted character of the thecal is carried further along the polypary 

 than usual ; the distal thecas are, however, of the true Halli type. 



Horizon ami Localities. — Grala-Tarannon Beds (especially zone of M. 

 turriculatus). 



S. Scotland : Belcraig Burn ; Ettrick Bridge End ; Dobb's Linn ; Caddon Water ; 

 Caddon Head, 6| miles W.N.W. Galashiels Station; Old Quarry, West Mains, 

 Old Cambns, Cockbnrnspath. Wales : Conway ; Nant Fuches-wen, and Rheidol 

 Gorge, Pont Erwyd. Lake District : Applethwaite Beck. 



Associates, etc. — Monofj. Halli is a fairly abundant fossil in the beds at the very 

 base of the Grala-Tarannon Series. In South Scotland it occurs in the Ra strifes 

 maximus band in the M. turriculatus zone associated with R. maximus, M. turriculatus, 

 M. runcinatus, il/. imclus, 3F. dextrorsus, Petaloy. altissimus and Climacog. extremus. 

 It also occurs with a very similar association in the M. turriculatus zone of Wales. 

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

 Lapworth, and the Authors. 



d 



c. Fragment in median portion of the poly- 



pary, preserved in low relief, and showing 

 apertural margins and short spines. 

 Belcraig Burn, near Moffat ; Upper 15irk- 

 hill Shales. Coll. Lapworth. 



d. Distal theca?, showing various appearances 



presented by compression. Belcraig - Burn, 

 near Moffat ; Upper Birkhill Shales (Band 

 of Rastrites maximus). Coll. Lapworth. 



e. Upper part preserved in full relief, lower 



part as a cast ; thecoe in relief show the 

 slightly retroverted apertures. Ibid. 



