174 SILURIAN RADIATA. 



the strong granules on the ridges corresponding to the inter- 

 cellular depressions of the true surface. Old specimens often 

 show a few thick irregular branching ridges on the surface as in 

 Neuropora. One young specimen is flat and oval, like the base 

 of Say's Favosites {Sfenopora) Lycoperdon, with which it might 

 be confounded if the posterior tapering of the cell-tubes was not 

 noticed; others are more elongate, conical; and the greater 

 number of specimens are irregularly undulated fragments, parts of 

 large expansions. The large, expanded, undulated and wrinkled 

 axis when the cells are rubbed off, resembles species of Stroma- 

 topora ; a careful search will usually show some remaining cells, 

 or by rubbing down the thin axis some of the cells of the under 

 side will be brought into view, when their comparatively greater 

 size and oblique ovate form will identify them. 



Very common in the slates of Mynydd Fron Frys ; five miles 

 west of Chirk ; Llansaintfraid ; Llanfyllin ; Blain y Cwm ; Cyrn y 

 Brain ; Coniston limestone of Coniston ; limestone of Mathyrafal ; 

 schists north end of Pen y Craig, above Glyn. 



(Col. University of Cambridge.) 



Ptilodictya costellata (M'Coy). 



Sp. Char. Corallum frequently dichotomizing at an acute angle ; 

 average width about 1^ line; surfaces gently convex, section 

 acutely elliptical, edges acute, with a rather broad lateral 

 margin, having a minutely granular non-celluliferous surface, 

 each face averaging from seven to ten longitudinal rows of 

 oblong or nearly oval cells, arranged in alternating lines (six 

 to eight rows in the space of 1 line, about four or five cells 

 in the same space in each row) ; cells about a third longer 

 than wide and half their length between the rows, and about 

 half their width between the cells of each row, giving the casts 

 the appearance of being regularly marked with longitudinal 

 sulci ; intercellular spaces very minutely granulose. 



The salcated appearance of the cast and the linear arrangement 

 of the cells easily distinguish this species from the Stictopora 

 {Ptilodictya) acuta (Hall. sp.). By grinding down a fragment I 

 have ascertained with certainty the presence of the flat internal 

 axis, which is however of great delicacy, and not to be seen in 

 the shale or sandstone specimens. 



Not uncommon in the slates of Llansaintfraid; calcareous 

 schists of Mathyrafal, and limestone of Girvan. 



[Col. University of Cambridge.) 



Ptilodictya fucoides (M'Coy). 



Sp. Char. Corallum forming very thin, flat, submembranous, 

 foliaceous, linear expansions, averaging 1^ to 2 lines wide, 



