DESCRIPTION OF CHALK CORALS. 259 
ridges were crenulated, the indentations agreeing with the grooves on the surface ; 
and the ridges themselves represented apparently successive surfaces ; but very 
thin slices taken vertically and transversely gave scarcely a trace of a concentric 
composition, nor did weathered stems, the most decayed exhibiting only a rough 
central hollow. In Isis Melitensis, on the contrary, enveloping layers can be de- 
tached mechanically, the exposed portions being fully ribbed ; and a cross-frac- 
tured segment presented many severed surfaces and concentric lines indicative of 
several more. Young and mature articular extremities had the whole area beset 
with these circles ; whereas an aged segment displayed a thick, outer layer, 
which had been moulded seemingly upon the edge of another of similar nature ; 
and being evidently ill-adapted for lateral motion, it exhibited possibly a precur- 
sory step towards a perfect consolidation. 
The visceral cavities were unequally and irregularly distributed in parts un- 
altered by age; and they were very readily distinguished from the articular pits 
on the side of a segment, by their elliptical form, shallowness, perpendicular sides 
and want of concentric ridges. The conical extremity of an incomplete segment 
(fig. 31) gave a deep, well-formed cavity with indentations around the edge, not 
more than half a line from the apex, proving the early production and rapid ad- 
vancement towards completion of the structure. The prevailing shape was an 
oval, the direction of the major axis generally conforming to that of the segment ; 
the length was half and the transverse measurement about $ths of a line ; and 
the dimensions at the bottom were nearly the same, making the sides almost per- 
pendicular. The floor was slightly concave or irregularly flat. The indentations 
around the periphery were seldom so exhibited that the exact number could be 
counted ; but they seemed to vary from eight to ten, differing very little, if really 
at all, from Jsis, which has eight tentacula. During the obliterating process, the 
cavities became gradually shallower, and the boundaries less defined, all traces 
of their previous existence being finally lost (fig. 30). 
Respecting the nature of the enveloping matter no evidence was afforded ; and 
not the slightest stain, indicative of its having existed, occurred in the chalk, 
whereas the form and even the texture of Amorphozoa are very often preserved in 
that manner. From this it may possibly be fair to infer, that it was of an eva- 
nescent nature, not very dissimilar from that of the crust of Antipathes, or per- 
haps wholly animal. 
The supposed root represented by figs. 33 & 33 a apparently belonged to a young 
specimen. The articular pit situated near the centre was concentrically furrowed 
