Vol. 65.] SUCCESSION ABOUND PLYNLIMON AND PONT EEWXD. 531 



The Scottish deposits afford the nearest faunal analogy with the 

 Plynlimon Stage, despite the great difference in the thickness of 

 the deposits and in their lithological characters. 



The Nant-y-M6ch Flags with Dicellograptus anceps and abundant 

 forms of the Orthograptus-truncatus group clearly correspond to 

 the fossiliferous lower part of the D. anceps zone of the Upper 

 Hartfell. In that region the base of the overlying Birkhill Shales 

 consists of a calcareous band crowded with Climacograptus scalaris 

 var. normalis, which underlies or forms the base of the zone 

 of Cephalograptus (?) acuminatus. This calcareous band seems to 

 correspond with the upper part of the zone of Glyptograptus 

 persculptus as developed near Fuches-gau, where the above-men- 

 tioned variety of Climacograptus is especially abundant. The base 

 of the Pont Erwyd Stage is, therefore, at an horizon at least as low 

 as the base of the Birkhill Shales. The two upper groups of the 

 Plynlimon Stage — the Drosgol Grits and the Bryn-glas Mudstoues — 

 must therefore correspond to the barren green shales and mudstones 

 which intervene in the Moffat country between the fossiliferous 

 shales with Dicellograptus anceps and the base of the Birkhill 

 Shales. As the Cardiganshire deposits are at least 2500 feet thick, 

 while the corresponding Scottish deposits are only about 12 feet 

 thick, it will be seen how great is the attenuation in passing from 

 one district to the other. 



In conclusion, I wish to express my thanks to many friends for 

 help received during the progress of the work, and especially 

 to Dr. Herbert Lapworth for much encouragement during the early 

 stages, and to Mrs. E. M. P. Shakespear and Miss G. L. Elles for 

 constant advice and assistance in identifying many of the grapto- 

 lites and in the difficult questions of nomenclature. I should 

 also like to tender my thanks to Prof. J. P. Ainsworth Davis, late 

 of the University College of "Wales, Aberystwyth, for hospitality 

 extended to me in the early stages, without which the work would 

 have been hardly possible at that time. 



YIII. Desceiption op Two New Species op Geaptolites. 



Monogbaptus atavus, sp. nov. (Fig. 18 a, b, c, & d.) 



1876. Monograptus tenuis (pars), C. Lapworth ' On Scottish Monograptidse ' 

 G-eol. Mag. dec. ii, vol. iii, p. 319 & pi. xi, probably figs. 3 a, 3 d, & 3 i. 



Polypary of unknown but great length, narrow, gently curved 

 or sometimes nearly straight in the distal portion. Near the 

 proximal end the thecse invariably lie on the convex side, but 

 distally they sometimes occur on the concave side. The width of 

 the polypary increases very gradually from *01 inch ("25 mm.) 

 at the proximal end to a maximum of about *05 inch (1-25 mm.), 

 a fragment of this width measuring 30 inches (76 cm.) having been 



