DESCRIPTION OF CHALK CORALS. 313 
mencements or inferior extremities of visceral cavities ; and lower down occurred 
larger openings, while the cylindrical edge gave regular-sized apertures. 
The cellular mouths of unthickened, axeal portions had a somewhat quincun- 
cial arrangement (fig. 6b) ; and they were strictly conformable to each other in 
situation, as shown by the position of the proximal edge ; the slightly abraded 
surface of such specimens gave also a similar distribution, or even one more de- 
cided (fig. 6c). Immediately below the nearly straight edge was generally a 
limited area (fig. 6 d), distinguishable likewise where the operculum existed, and 
when added to that plate, or to the open aperture, it made up an oval, which strictly 
agreed with that of an abraded cavity. This minor structure is considered of im- 
portance, as it proves that the cellular mouths were constructed at a particular 
period ; and its existence independently of the plate which occupied very many 
of the openings, supports the inference of that covering being truly an operculum. 
Immediately within the margin of the aperture (fig. 6b), or between it and the 
closing plate, was a well-defined furrow, and no clear indications of a progressive 
filling up from circumference to centre were observed. In a few cases the surface 
extremity of the cavity was wholly occupied by a layer similar to that which 
constituted the small space below the regular mouths ; but these instances were 
considered one of the abnormal developments common in zoophytes. The 
operculum exactly fitted the apertures, and, though of very frequent occurrence 
in unthickened branches, it was often absent. It will recall to mind Ellis and 
Solander’s delineation of the equivalent appendage in Myriopora or Millepora 
truncata', but in the recent coral it is horny, while in the extinct it was evidently 
calcareous. The slender form of the cavities in the middle of a branch (fig. 6 d) 
was apparently ill-adapted to digestive organs folded back upon themselves ; but 
possibly the long and confined character of the cell in that portion of the coral 
was an adaptation to a branched mode of growth; and the viscera were most 
probably not developed till the cavity attained its full diameter. In thickened 
stems covered with protuberances (fig. 6 e), the outline of the apertures was less 
regular than in axeal portions, being often round; and they had also occasion- 
ally a raised margin. In those points agreements will be found with M. Miche- 
lin’s enlarged delineation (op. cit. pl. 32. fig. 76), and the irregularities, if such 
they may be called, were due apparently to the deviations in plan of growth. 
The position of the mouth also, as shown by the proximal edge, varied through- 
out, according with that of the cell, or the nature of the protuberance, and not 
* Natural History of Zoophytes, or Lamouroux’s Exp. Méthod. tab. 23. f. 1-8. 
