SILURIAN RADIATA. 157 



the living Virgularia in the same state ; another simple fragment 

 about the same length has them half-expanded, being nearly 

 straight and oblique to the axis ; a third fragment has them quite 

 contracted, resembling a bit of narrow braid, exactly like the 

 contracted state of the recent Virgularia mirahilis ; this one shows 

 very plainly the transverse cell-ridging, rather less than 1 line 

 wide ; four pinnse in the space of 2 lines. 



In the slate at Lockerby. 



{Col. University of Cambridge.) 



Pyritonema fasciculus {M'Coj). 



I propose the above name for a singular fragment of a fossil 

 from the dark limestone of Tre Gil, nearly straight, about 2| 

 inches long, 4 lines wide, and 1~ line thick, and marked longi- 

 tudinally with coarse, thread-like ridges about the third of a line 

 in diameter, occasionally cut by small sharp transverse wrinkles, 

 the whole having some resemblance to an Ichthyodorulite ( Onchus 

 or Ctenacanthus) , with which I believe it was confounded by pre- 

 vious observers. On first seeing the specimen, I doubted this 

 reference, from observing that the ridges, instead of being merely 

 superficial^ thicker and more numerous at one end, as they should 

 be on this view, seemed equally thick at each end, and clearly not 

 in one plane, but those at the surface of one part plunging into 

 the mass and giving place to others emerging from it. Owing 

 to the skill and kindness of Mr. Anthony, of Caius College, two 

 sections for the microscope were prepared, which proved that 

 the whole mass was really a bundle of thread-like rods of silica, 

 corresponding exactly in diameter with the external ridges, the 

 sections of which exactly correspond with the others in the inte- 

 rior ; the siliceous fibres are solid, cylindrical, with slight occa- 

 sional transverse rugosities ; they are less than their own dia- 

 meter apart ; and the interstices show no organization under a 

 magnifying power of 330 diameters, the limestone being of a 

 finer texture and lighter colour than that of the matrix, as if there 

 had been originally a soft animal matter in the spaces between, 

 which kept out the coarser calcareous mud, but which became 

 filled with finer material by percolation on its decomposition. I 

 am therefore rather inclined to compare the fossil in question with 

 the Hyalonema of Gray, of which a short notice was published in 

 the Geol. Proceedings 1835, being, according to that naturalist, 

 a recent marine zoophyte allied to Gorgonia, called " Glass plant " 

 by the English at Canton. It has a long, thick, rope-like axis, 

 formed of a bundle of very long slightly twisted threads of pure 

 silica, held together by a little animal matter, the whole having 

 an external animal pulpy layer in which the polypes were lodged, 

 and which falls away at their death, leaving the siliceous axis of 



