﻿566 DR. T. GROOM AND MR. P. LAKE OK THE [NoV. I908, 



of fine and well- developed specimens of Orthis elegantula that 

 occur in them ; still higher the most characteristic fossils are 

 trilobites. 



In the western portion of our map (PI. LIII) the thickness of 

 this series is apparently very small, and there can be little doubt 

 that the upper beds are cut out by the Dolhir Fault. Similarly, in 

 the little stream about 250 yards east of Ty-isaf the lower beds only 

 of the series seem to be present. 



From this point eastwards the width of the band increases — an 

 increase which seems to be due, not merely to changes in the slope 

 of the ground and of the dip of the beds, but also to the appearance 

 of higher and higher beds of the series, until in the Glyn Valley a 

 thickness of some 500 feet is attained. 



From the Glyn Valley eastwards these beds can easily be traced 

 with frequent exposures to the streams of Nant lorwerth and 

 Ty-nant, a little to the east of which they end against the Cae-mawr 

 Fault. On the other side of this fault they are to be seen in the 

 valley of Craignant, but elsewhere exposures are very poor. 



In general, as has already been stated, the series consists of 

 sandstones and shales with calcareous bands. Fossils are very 

 abundant in places, and among the commonest and most characteristic 

 forms are Tetradella complicata, Phacops apiculatus, Trinudeus 

 concentricus, and Orthis elegantula ; but none of these are confined 

 to this horizon. Tetradella complicata and Trinudeus concentricus 

 are not found in the beds above, but they occur in the Teirw Beds ; 

 PhacojDS ujpiculatus ranges upwards into the Dolhir Eeds, while 

 Orthis elegantula extends from the Bryn Beds upwards into the 

 Fron-Frys Slates. But outside the Bryn Beds the specimens of Orthis 

 elegantula are always small and few in number; and so marked is 

 the difi'erence in size, that in this district a single well-developed 

 specimen of the species is almost sufficient to prove that the rock in 

 which it occurs belongs to the Bryn Beds. Orthis turgida^ Rajines- 

 quina ungula, and Triplesia spiriferoides are also characteristic. 



At one or two points the character of the beds and of the 

 fossils is distinctly peculiar, gasteropods and lamellibranchs being 

 particularly abundant. The most remarkable of these exposures 

 occurs in a small quarry, opposite the cottage called Bryn in I^ant 

 lorwerth. This quarry is cut in a thick mass of sandstone, from 

 which we have obtained the following fossils : — 



Monticulipora fibrosa, Goldf, 

 Moniiculipora lens, M'Coy. 

 Ortonia sp. 



Orthis elegantula, Dalra. 

 Plectambonites sericea, Sow. 

 Strophomena sp. 

 Ctenodonta varicosa, Salt. 

 Ctenodonta cf. transversa, Portl. 

 Modiolopsis postlineata, M'Coy. 

 Modiolopsis cf. securifwmis, Portl. 



Ortlionota prora (1) Salt. 

 Orthonota cf. rigida. Sow. 

 Bellerophon hilohatus, Sow. 

 Eaphistoma (Bquale, Salt. 

 Orthoceras ptolitum, M'Coy. 

 Tetradella complicata, Salt. 

 Asaphus sp. 



Homalonotus hisulcatus. Salt. 

 Fhacops apiculatus, Salt. 



A somewhat similar series of fossils, also in a sandstone-band, 



