STROM ATOPORELLA SOLITARIA. 211 



are from one to one and a half centimetres apart ; and is covered with small but 

 well-marked tubercles, some of which appear to be perforated at their apices. The 

 " mamelons " are formed by the upward bending of the skeletal laminas in such 

 a way as to form " astrorhizal cylinders," each of which encloses a vertical wall- 

 less canal forming the axis of a series of superimposed astrorhizae (woodcut, fig. 

 28). The opening of this axial canal is placed at the summit of a mamelou, and 

 the radiating canals diverge from this. The astrorhizee are, however, small and 

 circumscribed, and they do not become confluent by the anastomosis of the 

 terminal twigs of adjoining systems. 



Fig. 28. 



Fig. 28. — Vertical section through part of an astrorhizal cylinder of Stroma- 

 (oporella solitaria, Nich., enlarged twelve times. Middle Devonian, Gerolstein, 

 Eifel. (This is fig. 7, p. 56, where it is figured as helonging to S. eifeliensis, Nich.) 



As regards internal structure, the skeleton-fibre is moderately thick, and is 

 minutely porous, or even delicately canaliculated, though it does not exhibit the 

 marked tubulated structure of typical examples of 8. eifeliensis. Tangential 

 sections exhibit different appearances according as the section traverses an 

 interspace between two mamelons, or cuts across one of these eminences. In the 

 first case (Plate XXVII, fig. G) the cut ends of the radial pillars are seen, 

 separate or more or less confluent, and often showing central clear spots which 

 correspond with those seen in similar sections in 8. granulata, Nich., and 8. 

 Selwynii, Nich., and which indicate the presence of perforated surface-tubercles. 

 On the other hand, if the section traverses a mamelon (Plate XXVII, fig. 5), we 

 see in the centre the aperture of a transversely divided axial astrorhizal canal, 

 surrounded by concentrically disposed laminee with their uniting pillars. 



29 



