STROMATOPORA DISCOIDEA. 189 



inosculation of their terminal twigs with those of adjoining systems. As a result 

 of this, the entire surface (Plate XXIV, figs. 3 and 4) becomes mapped out into 

 innumerable small and irregular polygonal areas, bounded on all sides by the inter- 

 lacing astrorhizal canals. These areas, as seen in well-preserved examples, are 

 studded with minute rounded pores, which represent the openings of the zooidal 

 tubes. The astrorhizse are not arranged in vertically superimposed systems. 



As regards the internal structure of the coenosteum, the skeleton-fibre (Plate 

 XXIV, fig. 5) is minutely porous, and has a diameter of from i to ^ mm. 

 Tangential sections (Plate XXIV, fig. 6) show that the skeletal tissue is 

 completely reticulate, while the meshwork — owing to the thickness of the skeleton- 

 fibre and the small size of the zooidal tubes — is exceptionally dense. Such 

 sections show the repeatedly branched astrorhizal canals — often intersected by 

 transverse calcareous partitions ("astrorhizal tabula?") — and also the minute 

 rounded openings of the transversely divided zooidal tubes. Vertical sections 

 (Plate XXIV, fig. 7) show numerous thick and irregular radial pillars, which are 

 separated by the zooidal tubes, and are united at short intervals by stout and 

 irregular horizontal or oblique connecting-processes. The zooidal tubes are 

 intersected by numerous straight or curved transverse partitions or tabula?. 

 About five zooidal tubes, with their intervening pillars, occupy a space of 2 mm. 

 measured transversely. 



Obs. — The form of the ccenosteum in this species is remarkably constant, being 

 always that of a thicker or thinner disc-like expansion, with a basal epitheca. 

 Average examples vary from 6 or 8 cm. to 10 or 12 cm. in diameter, with a 

 thickness in the centre of from 1 to 5 cm., but hemispherical examples may be 

 considerably thicker than this. 



The surface-features in 8. discoidea are essentially conditioned by the form of 

 the astrorhizas, and are exceedingly characteristic. The astrorhiza? are exces- 

 sively branched, and also anastomose with their neighbours round their entire 

 periphery, there being thus no definite boundary between one astrorhiza and 

 those next it. Hence the surface is wholly cut up by the astrorhizal grooves into 

 irregularly shaped islands of ccenosteal tissue (Plate XXIV, figs. 3 and 4), which 

 are pierced by the minute rounded openings of the zooidal tubes. Even where 

 the surface has been much weathered it is generally possible to recognise under a 

 lens the peculiar lobulated structure thus produced. Though developed on the 

 surface of each successive lamina of the colony, the astrorhiza? do not form 

 regular vertical systems springing from axial wall-less canals, and there are 

 therefore no superficial eminences or " mamelons." 



The skeleton-fibre is minutely porous, but the pores are usually infiltrated 

 with some dark material, and appear therefore in thin sections as so many 

 opaque dots (Plate XXIV, fig. 5). This phenomenon is commonly seen in all 



