42 



BRITISH STROM ATOPOROIDS. 



lating form of the fossil, all obtainable tangential sections, as a matter of fact, run 

 partly through the horizontal laminse, and partly through the interlaminar spaces. 

 As regards the " concentric larninse " of Actino stroma clathratum, Nich., and of 

 similar forms, very different appearances are presented by tangential and vertical 

 sections respectively. The former show us, as above pointed out, that the concentric 

 lamina? are really formed by the inosculation and fusion of the radiating processes, 

 or " arms," thrown out by the radial pillars at definite and corresponding intervals. 

 It follows from this that the concentric lamina? are not, strictly speaking, " lamina? " 

 at all, but that they are really only a closer or looser reticulation of calcareous 

 fibres, penetrated by more or less numerous pores of various sizes and shapes 

 (Fig. 1, a). Hence, if we examine the surface of any concentric lamina in Actino- 



Fiq. l. 



Fig. 1. — Thin sections of Herniate/stroma Sckliiteri, Nich. A. Tangential section. B. Vertical section. 

 Enlarged twenty-four times. The tangential section passes along the plane of one of the concentric 

 lamina;, and shows the cut ends of the radial pillars with their axial canals (p p), the extension of 

 the canals into the lamina; (c c), and the pores formed by the inosculation of the horizontal pro- 

 cesses or "arms" (o o). The vertical section shows the axial canals of the pillars (p p) and the 

 extensions of these canals horizontally into the concentric lamiua; (e c). Devonian, Herhorn, near 

 Paffrath. 1 



stroma clathratum, either by looking at the actual surface or by studying the plane 

 of a concentric fracture, or if we take a properly prepared tangential section, we 

 can observe numerous minute pores passing through the lamina, and placing the 

 interlaminar space below the lamina in direct communication with the interlaminar 

 space above the same. These pores are most readily recognised as being truly 

 " pores," if we have under observation such forms as any of the true Stromatoporce, 

 in which the general skeleton is reticulated and continuous, but we cannot refuse 

 this name to the wider, more open, and more irregular meshes formed by the union 

 of the horizontal " arms " in the typical Actino stromata (Plate I, figs. 8 and 10). 



1 I take this opportunity of expressing my sense of the very admirable manner in which Mr. 

 Charles Eerrier, F.L.S., has engraved on the wood such highly trying subjects as the thiu sections 

 figured in this work. 



