108 BRITISH STROMATOPOROIDS. 



10, 11) which is crossed by more or less numerous curved tabulae, and which gives 

 off lateral tabulate branches. Judging from the few examples which I have seen 

 in which the ends of the branches are perfectly preserved, it would appear that the 

 main axial tube terminates at the end of each branch in one, two, or more 

 large-sized apertures. The lateral divisions of the main axial tube, however, sub- 

 divide and give off numerous small zooidal tubes, which are continued to the sur- 

 face, and which seem to be only sparsely furnished with tabulae. Growth of the 

 ccenosteum is effected by the formation of successively formed convex layers, which 

 are much thicker over the growing ends of the branches than elsewhere (Plate 

 VIII, fig. 10), and which give rise in thin sections to a series of curved concentric 

 lines, the convexities of which are turned towards the distal end of the colony. 

 There are, however, no true concentric laminae, nor can any definite radial pillars 

 be recognised. The skeleton is continuously reticulated, and the sclerenchyma 

 is everywhere traversed by innumerable delicate tubuli, which run parallel 

 to the zooidal tubes (Plate VIII, fig. 14). In tangential sections (Plate VIII, fig. 

 13), the cut ends of these tubuli are seen, sometimes as minute rounded apertures, 

 sometimes as dark dots (Plate XI, figs. 5 and 6), according as the tubuli are empty 

 or are infiltrated with oxide of iron. Though in the main running parallel with 

 the zooidal tubes, the tubuli frequently branch and anastomose with one another. 



Stachyodes verticillata shows some curious points of resemblance to certain of the 

 Stylasteridce. Thus, longitudinal sections of Distichopora, taken in the median 

 plane of the ccenosteum and dividing the pore-tubes lengthways, show phenomena 

 in many ways resembling those presented by Stachyodes (Plate IX, fig. 5). This 

 resemblance is particularly marked as regards the microscopic tubuli of the 

 skeleton-fibre of both these types. If Stachyodes stood quite alone there might be 

 some ground for regarding it as an ancient type of the Stylasteridce. It has, 

 however, strongly marked relationships to Idiostroma, Winch., and through this 

 with the whole group of the Stromatoporidce proper. It agrees entirely with the 

 cylindrical types of Idiostroma as regards the possession in the interior of the stems 

 of a tabulate axial tube, from which spring secondary lateral tubes, which are also 

 tabulate. In fact, the essential point by which it is separated from Idiostroma is 

 only the characteristic tubulation of the skeleton-fibre. 



