SUB-GENUS PRASOPORA. 213 



between the results of external and internal observation may 

 be that the apertures of the small tubes on the surface are in 

 general concealed by their overgrowth with a calcareous mem- 

 brane. A few, however, of the minute openings of the inter- 

 stitial tubes, may usually be detected in one part or another of 

 the surface of the corallum by a sufficiently careful search with 

 a one-inch objective. 



From the great thinness of the expanded corallum, it would 

 naturally be supposed that the colony was parasitic upon 

 foreign bodies ; and none of my specimens would disprove this 

 supposition, as in all the upper surface alone is shown, and the 

 under surface is buried in the matrix. Thin vertical sections, 

 however, so far as I have seen, do not show any foreign body 

 beneath the thin crust of corallites, but show that the under 

 surface is covered with a thin epithecal membrane. 



The general form and surface-characters of this species are 

 quite peculiar ; but the internal structure is even more dis- 

 tinctive. Tangential sections (PI. IV. figs, i c and d) show 

 that the corallum is composed of two distinct sets of tubes, 

 which differ not only in size, but, very markedly, in their 

 internal structure. The larger tubes, instead of showino; the 

 polygonal form which they have at the actual surface, are oval 

 in shape, and are therefore only in contact at very limited 

 points of their circumference. They possess thin but quite 

 well defined walls, and their tabuke are imperfect, appearing 

 (in sections of this kind) as if perforated by crescentic apertures 

 on one side. The interspaces left between the large tubes are 

 occupied by a series of small corallites, which never completely 

 isolate any single large corallite, and of which no more than a 

 single row is ever present between any given pair of the large 

 tubes. These small corallites are very variable in size, and are 

 angular in their shape. In vertical sections (PI. IV. fig. \c), 

 the differences between these two sets of tubes are very clearly 

 marked;, quite apart from the difference in size. The large 

 corallites are now seen to be furnished with purely unilateral 

 tabulae, which are convex, with their convexities turned towards 



