3i8 TABULATE CORALS. 



hemispherical when fully grown. The lateral margins of the 

 corallum are, however, very often extended and comparatively 

 thin, the chief elevation of the corallum being central, and there 

 is often a curious funnel-shaped depression at the summit of the 

 colony, the nature of which I do not understand. Adult coralla 

 reach a diameter of from one and a half to three inches, with a 

 height of three-quarters of an inch to an inch. The characters 

 of the surface are very like those of the typical M. petropolitana, 

 Pand., especially in the presence of low elevations or monticules, 

 composed of corallites which seem to be slightly larger than the 

 average. Thin sections show that the corallum also, as in M. 

 petropolitana, is composed of very thin-walled corallites, the 

 walls of which are undistinguishably united, and are not thick- 

 ened towards the surface ; while two distinct sets of tubes, of 

 different sizes, form by their intermixture the entire colony. 

 Tangential sections (PI. XIII., fig. 4 6) clearly exhibit both 

 groups of corallites, both being angular in shape, and the 

 small tubes being situated at all the angles left between the 

 larger ones. The latter are seldom completely surrounded 

 by small tubes, though this does sometimes occur. Very 

 characteristically the large tubes are arranged in stellate 

 groups or rosettes, consisting of four or five tubes ; and 

 the central point where these meet is marked by a thick- 

 ened rounded or quadrate body, which exhibits commonly a 

 median tube. Similar structures are often developed at the 

 points where the small corallites abut against the larger ones, 

 and their appearance is precisely similar to that of the 

 "columns" oi Dekayia, or the spines of certain of the Monti- 

 culiporcs as seen in transverse section. I cannot say, however, 

 that I have ever detected spines on the surface in this species, 

 though I cannot doubt that the structures to which I have just 

 referred are peculiarly modified corallites. Vertical sections 

 (PI. XIV., fig. i) show that the corallites, instead of being 

 reclined towards the margin of the colony, are everywhere 

 nearly perpendicular to the epithecal plate ; and they further 

 show very striking differences in the structure of the large 



