﻿Vol. 6 1.] AEEIST1G PAWB AND MOEL LLYFJ5TANT. 617 



after the manner of the Houghton "Whetstones of Yorkshire. 

 Fossils are, as a rule, fairly abundant, the most characteristic of 

 them being the beautiful little Shumardia. Slabs with ten or a 

 dozen specimens of Asaphellus Homfrayi, Salt, (of narrow type) 

 are not uncommon, and Agnostus Siedenbladhi, Linn., and Macro- 

 cy stella Maria, Call., may also be obtained from most exposures. 

 The fauna obtained is as follows: — 



Asaphellus Homfrayi, Salt. ! Bikelocephalus. 



Shumardia salopiensis (Call.), cf. 



celandica, Moberg. 

 Ogygia (Niobe) scutatrix, Salt. 

 Olenus(Paraboli)iella) triarthrus, Call. 

 Agnostus Siedenbladhi, Linn. 

 Symphesurus Croftii, Call. 

 Remopleurides. 



Oheirurus. 



Conularia Homfrayi, Salt. 



Theca operculata, Salt. 



Bellerophon multistriatus, Salt. 



Gtenodonta. 



Lingula cf. Nicholsoni, Call. 



Acrotreta (Obolella) Sabrin<B, Call. 



Holometopus. j Protospongia-$$\c\\[es. 



There are also abundant, branching, carbonaceous markings 

 which are often regular enough to suggest graptolites. but are not 

 sufficiently well preserved for identification. 



The best localities and collecting-grounds are the east-and-west 

 stream-sections south of Amnodd-wen and north of Amnodd-bwll 

 respectively. The latter is particularly good, and has yielded the 

 best specimens of Shumardia and many beautiful slabs of Macro- 

 cy stella Marice, Call. The gorge known as Ceunant-y-gareg-ddu 

 presents an excellent exposure of the lowest beds, and has yielded 

 examples of all the fossils recorded, except Remopleurides. Remo- 

 pleurides, indeed, seems to be exceedingly rare, and is only repre- 

 sented by Hr. Williams's one specimen, which came from Amnodd- 

 wen. The best Asaphellus-slabs were obtained from the bed of 

 the stream, which crosses the road about 250 yards south of 

 Hafotty Filltirgerig. 



As will be seen from the foregoing list, the ShumardiaShales 

 may be exactly correlated with the main fossiliferous horizon of 

 Sheinton, and with beds overlying the Post-Office Flags at Pen- 

 morfa. The highest Shumardia-Beds of the latter locality, which 

 are there exceedingly prolific, do not seem to be represented here, 

 and the overlying Angelina-Beds are almost certainly absent, 

 probably by reason of the succeeding unconformity. 



Here, then, ends the series which British geologists are in the 

 habit of calling Cambrian, and, in leaving it, I would remark 

 upon the exact correspondence between each of the divisions and 

 its homotaxial equivalent at Portmadoc. Each fossiliferous band, 

 each change iu the character of the sediment, can be recognized ; and, 

 although the total thickness must be greater at Arenig than at 

 Tremadoc, I think that the difference is due rather to subsequent 

 packing by earth-movement than to diversity in original deposition. 



Above the Shumardia-Beds, Asaphellus-Fl&gs, or Bellerophon- 

 Beds, is a surface of unconformity. This is not a great dis- 



