and species of Silurian Radiata. 287 



branous, very thin, marked with small, close, concentric or 

 forward-curved undulating wrinkles; tube-cells pyriform ob- 

 lique, three or four times longer than wide, narrowed poste- 

 riorly, anterior ends of the adjacent cells convex rhomboidal, 

 wdth a round perforation nearest the anterior edge; cells 

 separated by depressed lines, which (when finely preserved) 

 show one or two alternating rows of small cellules, about 

 seven or eight cells in a space of 2 hues. 



The broad fohaceous expansions of this species readily distin- 

 guish it from any of those described by Mr. Hall, as well as the 

 very narrow depressed spaces between the cells on the surface, 

 and the broad rhomboidal, convex portion of the cells suiTound- 

 ing the aperture. Finely preserved casts of the surface show 

 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 {Stenopora) Lycopej-don, 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 Sti-oma- 

 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. 



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 i*ows, 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. 



