Tol. 52.] FROM THE CARBONIFEROUS LIMESTONE. 441 



the mural pores are frequently in part filled with iron pyrites, and, 

 as the walls are thin and delicate, the preparation of thin sections 

 is rendered difficult. 



Specific Characters. — Corallum small, quite free and without any 

 trace of attachment. In its fullest development it consists of four 

 depressed corallites, starting, on the same plane, from a common 

 centre, nearly at right angles to each other; in some specimens 

 only three corallites are present, in others two ; while simple forms 

 of but a single corallite are also not uncommon. In the larger 

 forms with four corallites (PI. XXIII. figs. 1, 2, 3 & 4) the central 

 portion of the base is flattened, and the corallites extend from it at 

 first in a nearly horizontal direction, and then they gradually curve 

 upwards and outwards, so that their apertures are somewhat oblique. 

 The free portions of the corallites are subcylindrical, in section 

 varying from circular to elliptical. The corallites are not always 

 of the same length ; usually the two forming the transverse axis are 

 subequal and shorter than those of the longitudinal axis, and in this 

 axis one corallite is, as a rule, markedly longer than the other 

 (PI. XXIII. figs. 3 & 4). The outer surface of the corallum is marked 

 by distinct ridges, with sharp continuous edges, which, in the distal 

 or free portions of the corallites, are longitudinal, straight, or slightly 

 wavy, and subparallel, but in the central portions of the corallum, 

 both on the basal and upper surfaces, they are usually broken up 

 into short, discontinuous, somewhat labyrinthine patterns. In the 

 longitudinal furrows between the surface-ridges there are rows of 

 subcircular mural pores, disposed in alternating series, which pene- 

 trate the wall direct and open into the visceral chamber of the 

 corallites. These mural pores are not limited to the free lateral 

 portions of the corallites, but they are present in the basal portions 

 as well, some connecting the visceral chambers of adjacent corallites, 

 while others communicate with the exterior (PI. XXIII. figs. 6, 9 & 

 10). The mural pores are but seldom exposed to view on the outer 

 surface of the coral, unless when it is weathered or rubbed down. 



The calice ; or the interior of the corallite, is conical or turbinate 

 in form ; the inner surface of the wall shows the rows of mural 

 pores, and the spaces between these are nearly smooth or marked 

 l)y slightly impressed lines, but there are no indications of granu- 

 lations, tubercles, or septa, to correspond with the ridges of the 

 exterior surface (PI. XXIII. fig. 12). The apertural margins of the 

 corallites are thin and slightly crenulate from the projection of the 

 edges of the wall-ridges. 



In the specimens which consist of only two corallites, these are 

 joined end to end, so that, viewed laterally, the forms appear as 

 short, nearly cylindrical, curved pipes, with a circular aperture at 

 each end (PI. XXIII. figs. 5 & 6). The internal form of these coral- 

 lites is well shown in the casts from the Lower Culm (fig. 6). From 

 the central portion of these twin forms of corallum one or two 

 additional corallites are occasionally developed, but in several ex- 

 amples there are no traces of any lateral buds, and the corallum 

 appears to have permanently remained a twin form. In the simple 



2h2 



