166 SILURIAN RADIA.TA. 



averaging 5 lines in height and width, radiated with about 

 forty-five or fifty lamellar sulci ; average internal casts nearly 

 5 lines wide and 2 lines high, radiated by the deep slits of 

 twenty-five strong primary lamellae (six or seven in 3 lines at 

 the margin) extending to the centre, where they are united 

 and irregularly complicated ; between each pair of these slits is 

 a row of small, close, irregular, numerous pits (representing 

 papillae of the secondary lamellae), about three in a space equal- 

 ing the distance of the lamellar slits apart. 



I frequently find this common species confounded with the 

 P. Una (Lonsd.), which in form and size it resembles, but from 

 which it difi'ers completely in the primary lamellae being simple 

 plates extending to the complicated centre, leaving deep slits in 

 the cast, which therefore has but one set of rows of pits instead of 

 two ; while in P. bina these lamellae are replaced by a row of large 

 papillae, leaving a row of large pits in the cast, extending but a 

 little way towards the centre. The secondary row of lamellar 

 papillae (and pits on the cast between the slits) is much smaller, 

 closer and less regular in the present species than in the P. hina. 

 Some of the casts present a depression in the smaller end, into 

 which a process of the solid part of the apex must have pene- 

 trated. Between the lamellar sulci in some damaged specimens, 

 obscure traces of cells seem doubtfully visible under the lens ; — 

 if these should prove to be vesicular plates, the species should be 

 transferred to the genus St7^ephodes, thus removing it still further 

 from P. binaj in which there is nothing of the kind. 



Var. a. gracilis. 



Certain casts a little longer and of a much more elongate form 

 than the above, being about as high as wide, and having about 

 thirty to thirty-six primary lamellar slits at a diameter of 6 or 

 7 lines, and an equal number of rows of close, small puncta, re- 

 quire separate mention, though I do not see that they differ in 

 other respects. 



Common in the schists of Pen lar Llandovery; Mathyrafal ; 

 Llansaintfraid and Castel Craig ; fine sandstone of Alt y Anker. 



[CoL University of Cambridge.) 



Cyathaxo7iia Siluriensis (M'Coy). 

 Sp. Char. Corallum elongate conic, about 5 lines long and 2 

 lines in diameter at that height from the base; strong central 

 axis nearly one-third of the diameter; sixteen or seventeen 

 strong radiating lamellae, each extending from the axis to the 

 outer wall, before reaching which it bifurcates, leaving a tri- 

 angular interlamellar space about equal in width to the di- 

 stance between the adjoining lamellae ; surface coarsely ridged 



