28 PROF. E. J. GARWOOD AND MISS E. GOODYEAR [vol. lxxiv, 



often a tendency towards an increase in the diameter of the cells 

 along this junction-line, similar to that whioh characterizes the 

 basal layer of the thallus. 



Occasionally, small elongated fusiform spaces (PL VI, fig. 1) 

 occur in the thallus, filled with crystalline calcite ; these lie with 

 their longer axes parallel to the direction of the cell-threads — 

 they vary in length from mere specks up to 300 ^u, and their 

 greatest width is about 50 /•. Similar spaces have been described 

 by Kothpletz in S. gotlandica as sporangia, but it is very doubtful 

 Whether such an interpretation can be placed on these spaces in the 

 Old lladnor species ; they suggest, rather, spaces due to the 

 irregular solution of portions of the cell-walls, since the cavities 

 interrupt the continuity of the cell-walls and are not definitely 

 enclosed by them. 



In cross-sections the structure appears as closely-packed poly- 

 gonal cells, having a nearly uniform diameter of 17 u. This 

 species resembles most closely S. gotlandica Roth pie tz from the 

 Sphcerocodiam Limestone of Southern Gotland — an horizon con- 

 sidered by Dr. H. Munthe to be the equivalent of the Aymestry 

 Limestone of this country. The cells in S. gotlandica are, how- 

 ever, much larger, measuring from 25 to 35 /i, while the thallus 

 seldom attains a greater diameter than 3 cms. The size of the 

 cells in S. gracilis approaches very closely to those of some species 

 of Litliotliamnion, in which they range from 6 to 20/<. 



EXPLANATION OF PLATES I-VIL 



Plate I. 



The base of the limestone, Dolyhir Quarry, showing included angular frag- 

 ments of Pre-Cambrian grit and tabular growths of Favosites. The 

 higher beds show white nodules of Solenopora, as also the blocks in 

 the foreground. 



Plate II. 



View of Old Radnor Hill, looking north-eastwards across the railway from the 

 south side. The plantation on the right of the road in the centre of 

 the picture marks the old limestone-quarry, at the back of which 

 runs the east-and-west fault. The Pre-Cambrian conglomerate out- 

 crops are seen on the slope behind Hillhouse Farm. 



F.F, = Boundary-faults. R.R. = Railway. 



T. —Thrust-plane in limestone. H. = Hillhouse Farm. 



S.R. = Northern end of Stanner Rocks. Y. == Pre-Cambrian Quarry, Yat 



Str. = Entrance to Strinds Quarry. Wood. 



Plate III. 



View of Yat-Hill Quarries, looking north-north-westwards from the road to 

 Strinds Farm, behind the old quarry, Dolyhir, showing the fault- 

 plane bounding the eastern side of the quarry. One of the parallel 

 thrust-faces, bringing up a patch of Pre-Cambrian grit in Yat 

 Quarry, is seen on the left in the distance. 



