132 BRITISH STROMATOPOROIDS. 



by horizontal processes or " arms," which are given off with great regularity in 

 radiating whorls, the result being the formation of an angular mesh work, which 

 in tangential sections (Plate XII, fig. 2, and Plate XIII, fig. 1) has a close 

 resemblance to the structure of a hexactinellid sponge. The angular pores 

 formed in this way served for the emission of the zooids, and definite tabulate 

 zooidal tubes are not present. 



Obs. — Actinostroma clathratum, Nich., is of very variable outward form, and 

 has also a wide range as regards size. Small specimens are usually globular, sub- 

 globular, or pyriform ; large examples are mostly hemispherical or irregular in shape. 

 The under surface is generally, if not always, non-epithecate, and the upper surface 

 is destitute of " mamelons. ; " Very generally the coenosteum is composed of 

 successive thick, concentric strata (" latilaminas ") ; but in some instances this 

 structure is not observable. In the former case the radial pillars are continuous 

 from the bottom to the top of each latilamina at least. In the latter case the 

 pillars run continuously for apparently indefinite distances. 



In some specimens " astrorhizse " cannot be detected at all, but they are by 

 no means universally wanting in this species, as stated by Bargatzky. I have 

 now found them to exist in many of the German specimens, and they seem to 

 be always, or almost always, present in British specimens. In the Dartington 

 specimens they are even numerous. Their individual development is, however, 

 an imperfect and irregular one, and they do not appear to be usually superposed 

 in vertical series in successive interlaminar spaces. Hence the coenosteum is not 

 traversed by conspicuous vertical astrorhizal canals, and, for the same reason, 

 the surface is free from " mamelons." 



The surface, in well-preserved examples, is studded with minute rounded 

 tubercles, representing the free ends of the radial pillars (Plate II, fig. 11), and 

 sometimes exhibits also the radiating connecting-processes between them and the 

 intervening zooidal pores. In some dolomitised specimens the skeletal network 

 becomes dissolved out of the matrix towards the surface of the mass, leaving stellate 

 or rounded pores which represent the pillars and their radiating " arms " 

 (Plate I, fig. 5). 



In spite of the great variability in external form, the general type of internal 

 structure is in this species very constant. The radial pillars are stout, usually 

 rounded, but in some forms subangular. Cross sections of the pillars usually 

 show a minute axial canal (Plate I, figs. 10 and 13), but this cannot always be 

 made out. The horizontal " arms " are very regularly produced, and give rise 

 by their union to a regular " hexactinellid " network, the zooidal pores being more 

 or less angular in form (Plate XII, fig. 2). 



On an average there are generally about three radial pillars and as many 

 concentric laminas in the space of a millimetre. It must be borne in mind, 



