DESCRIPTION OF CHALK CORALS. 247 
distinct expansion at the base, but the interior had rather more prominent lamell 
than in the former instance. This young cavity was so intimately connected with 
the structure of the mature coral, that no doubt could be entertained of its having 
been produced by the polype which inhabited the latter. No evidence was ob- 
tained whether a marginal development was effected progressively as the side of 
the parent was perfected, or whether it was a pallial production: doubts may also 
be entertained respecting the origin of the example, figure 7, as it might have 
been formed in the mantle, or have sprung from an ejected ovum accidentally 
settled on an exposed side. Whatever the source may have been, it is plain that 
such young cavities were of infrequent occurrence, and should be regarded as 
exceptional rather than essential productions. 
Three other corals are represented in Tab. XVIII. (figs.11,12,13), as they ex- 
hibited characters somewhat dissimilar from those mentioned in the preceding 
paragraphs. One of them (fig. 11) differed from the specimens previously noticed 
in the lamellz being knife-shaped, regular in outline, and almost in contact at the 
periphery of the coral (fig. 11a); and not thin throughout, waved, and separated at 
the circumference of the cavity by clear intervals. It presented also, as before 
mentioned, traces of rounded lobes in front of the broadest lamella. Externally, 
the ribs were visible only in the upper half of the surface, the lower being smooth, 
and partially coated by Bryozoa: the internal structure could not be ascertained. 
Mr. Dixon’s collection contained two other clear examples of similar lamelle ; 
but the specimens (fig. 12) were much more slender than that just noticed, 
bearing about the same proportions to it that figure 6 bears to figure 2. Their 
internal composition was also not ascertained; but their exterior was smooth 
nearly to the upper margin, and bore evident proofs of considerable outer addi- 
tions ; attached zoophytes and apparently annelides were likewise overlaid by the 
polype secretions. The three corals thus far characterized were believed to be- 
long to Monocarya, and should the internal struetures warrant a specific separa- 
tion, they might perhaps be distinguished by the term Monoc. cultrata. The 
other fossil (fig. 13), obligingly lent by Mr. Tennant, F.G.S., agreed in the 
outline of the lamellz with ordinary specimens of M. centralis ; but the sides of 
the plates were nearly smooth, and there was an almost total want of a central 
reticulation. This structure has been already stated to vary considerably, but 
chiefly in proportion to the diameter of the specimen. Mr. Tennant’s coral 
exceeded in this respect the largest examples of M. centralis which were ex- 
amined; yet it had a less definite centre than the lower portion of figure 3. From 
2K2 
