SUB-GENUS HETEROTRYPA. 121 



among the ordinary callces, and sometimes aggregated together 

 to form distinct maculce. The walls of the corallites, so far as 

 appearances go, are undistinguishably fused together, their sub- 

 stance being perforated by numerous minute tubules, which 

 usually form a single row between adjoining corallites. In 

 the axis of the stems the corallites are thin-walled, but their 

 walls become excessively thickened as they bend outwards to 

 reach the surface. Complete horizontal tabulae are developed 

 in moderate numbers, both in the axis of the branches, and 

 also in the outer portion of the corallites. 



Obs. — This very remarkable species, which is so abundant 

 in the Carboniferous rocks of Britain, exhibits structural fea- 

 tures of an unusual character. In its general form (PI. III. 

 fig. i) it is dendroid, and the most constant of its superficial 

 peculiarities is the great thickness of the walls surrounding the 

 oval or sub-polygonal calices (PI. III. fig. \d). Some speci- 

 mens exhibit an apparently smooth surface, and none have 

 " monticules " in the proper sense of the term ; but, when well 

 preserved, the surface usually exhibits numerous prominent 

 blunt spines (PI. III. figs. \a and i^), which give a character- 

 istically rough appearance to the exterior, as viewed under the 

 lens or microscope, and which surround the mouths of the 

 calices in a single row. In some specimens, the surface shows 

 no other apertures than those of the proper calices (PI. III. 

 fig. id), or, perhaps, only an occasional minute opening. In 

 other specimens, again, we find a moderate number of much 

 smaller openings intercalated amongst the normal calices ; and 

 the former are also not unusually aggregated into star-like 

 groups or "macular." In the normal form of the species, how- 

 ever, these interstitial openings are never excessively devel- 

 oped ; and except where a " macula " may be situated, there is 

 never more than a limited number of tubules at the angles of 

 junction of the larger tubes. 



As regards internal structure, tangential sections (PI. III. fig. 

 i^) show that the walls of the corallites are seemingly complete- 

 ly fused together, no sign of any line of demarcation between 



