part 1] ON THE GEOLOGY OF THE OLD RADNOR DISTRICT. 27 



originally gave rise to the valley now followed by the railway. 

 although the present gorge may well owl- its existence to an over- 

 . flow stream from one of the numerous post-Glacial lakes, relics 

 of which form conspicuous features in the district. 



V. Description of Solenopora gracilis, sp. nov. 

 (PI. VI, figs. 1 &2.) 



This species differs from all other known examples of the genus 

 Solehopora, on account of the massive character of the thallus 

 (some specimens measuring more than 17 cm. in diameter) and 

 the minute size of its cells. It occurs in the usual nodular form 

 characteristic of the other species of the genus; hut, in many cases, 

 the outer surface has been destroyed by the solution and crushing 

 which the limestone in the Old Radnor district has undergone. 

 The internal structure is too minute for any trace of the cells to 

 be observed in hand-specimens, even with a good lens; occasionally, 

 however, indications of the concentric lines of growth may be 

 traced. Freshly-fractured surfaces display the porcellaneous tex- 

 ture characteristic of this genus. 



Specimens extracted from the heart of the limestone are of a 

 deep indigo or purple colour ; some, however, are blotched with 

 patches of a paler tint where they have been partly bleached by 

 percolating water, and they then exhibit irregular concentric 

 banding. When exposed to weathering, the whole of the thallus 

 becomes bleached, and these organisms then appear as conspicuous 

 white spots on the surface of the limestone. 



Under the microscope, longitudinal sections show a series of 

 closely-set sub-parallel tubes, or cell-threads, having an average 

 diameter of 17 /.*. The walls of these tubes are straight and thin, 

 but the specimens in the Old Radnor limestone have undergone 

 such complete recrystallization that their detailed structure is not 

 always easy to ascertain. The length of the cells averages 25 fx. 

 In places where the organism is in contact with a foreign body, 

 the cells are wider; they tend to be more irregular in their disposi- 

 tion, and may then reach a diameter of 20 to 25 /x. Rothpletz 1 

 differentiates the portions of the thallus characterized by these 

 two classes of cells into ' perithallium ' and 'hypothallium ' respec- 

 tively. In our species the larger cells are not limited to the 

 first-formed layers of the thallus, but are liable to occur wherever 

 the growth of the organism is interrupted bv the introduction of a 

 foreign body (PI. VI, fig. 2). 



These longitudinal sections also exhibit numerous concentric 

 lines of growth, which occur at irregular intervals, and are fre- 

 quently marked by a discontinuity in the growth of the cell- 

 threads on each side of them. They appear to mark periods 

 when the growth of the organism was interrupted, and there is 



1 ' Ueber die Kalkalgen aus dem Obersilur Goi lands ' Sver. Geol. Under- 

 sold!. Ser. Ca No. 10 (1913) p. 7. 



