Vol. 56.] SILURIAN SEQUENCE OF RHAIADER. 135 



Horizon. — The base of the Rhayader Pale Shales, where it 

 forms the commonest fossil. Found in conjunction with Monoyrap- 

 tus crassus, Lapw., M. pandas, Lapw., M. Becki, Barr., etc. 



[With reference to the question asked by Mr. Marr in the course 

 of the discussion which followed the reading of this paper, I have 

 inserted the following description of 



DlPLOGRAPTUS MODESTUS, LapW. 



1876. Diplograptus modestus, Lapw., Lapworth, Catal. Western Scot. Foss. pi. ii, 

 fig. 33. 



1877. Diploqraptus eonfertus, Nich., Lapworth, ' Graptolites of County Down ' 

 Proc. Belfast Nat. Field Club, App. pi. vi, fig. 8. 



1897. Diplograptus modestus, Lapw., Perner, ' Graptolites de -Boheme ' pt. iii, 

 p. 5 & pi. x, fig. 8. 



This species has been figured in the publications mentioned above, 

 but without description. The following are the essential characters 

 of the species, as noted in the MSS. of Prof. Lapworth :— 



Rhabdosoma simple, diprionidian, with parallel margins, and 

 an obtusely pointed base ; from 1 to 1J inches in length, and 

 l 1 ^ inch in width. There is an extension of the virgella of J to J 

 inch long. Virgula prolonged beyond the youngest hydrothecoe for 

 a length of \ or even \ inch. 



Hydrothecse 26 to 30 in the inch, inclined at an angle of about 

 30° to the axis of the rhabdosoma, quadrangular in section or 

 very slightly rounded. Apertural margins somewhat concave, at 

 right angles to the axis of the rhabdosoma and prolonged in an 

 acute denticle. Outer margins of hydrothecse slightly convex. 



This species is recognizable by its comparatively small size, its 

 breadth, the concave apertural margins, and the convex margins 

 of the hydrothecae. 



Horizon. — Lower Birkhill. Very common in the Diployraptus- 

 vesiculosus zone at Bobb's Linn, etc., Moffat ; and in the D.-modestas 

 Flags of the Gwastaden Group, Ehayader. — December 8th, 1899.] 



EXPLANATION OF PLATES VI. & VII. 

 Plate VI. 



Detailed Vertical Section of the Strata of the Rhayader Sequence, on the scale 

 of 300 feet to the inch. 



Plate VII. 



General Geological Map of the Ehayader District, on the scale of 1 J inches to 



the mile. 



Discussion. 



Mr. Mare congratulated the Author on his paper, his map, and 

 the lucid way in which he had placed his results before the Society. 

 He had followed his father, not only in describing Valentian rocks, 

 but in choosing a particularly difficult region for description. The 

 Author spoke of the Diplograptus-modtstus Beds. He (the speaker) 

 would be glad to know whether that species had ever been described. 



