DESCRIPTION OF CHALK CORALS. 257 
plain that, for a certain period, the segments of the axis were occupied by abdo- 
minal structures, and that the hollows were gradually obliterated. No vestige 
of a crust could be detected, though no doubt of an investing animal substance 
could be entertained. One indication of an overlaid interval was noticed. 
On comparing the composition of the chalk fossil with that of Isis Hippuris, 
points of resemblance will be found in the general form and furrowed surface of 
the segments ; in the side offshoots issuing from calcareous portions of the axis, 
also in two occasionally proceeding from one termination ; and it is believed that 
the intervals in both zoophytes were equally overlaid by calcareous additions. 
The differences consist in the fossil having more decided articular surfaces, with 
much less of interval ; and in the all-important distinction of the segments being 
penetrated by abdominal cavities ; while it is universally admitted that in Isis 
they are confined to the investing layer. 
As respects Isis Melitensis, an agreement exists in the articular surfaces and 
in the construction of the segments ; but that coral is far too imperfectly known 
to the compiler of these memoranda for an opinion to be hazarded of its full 
characters, though he doubts if the generic determination should be received. 
From the difference in the position of the abdominal cavities, it is conceived 
the chalk fossil cannot be regarded as a species of Isis, or of any other genus of 
articulated Anthozoa, known to the describer, He therefore suggests, that it 
might be distinguished by the name Avogaster (a&wv axis, yaornp venter), in con- 
sequence of the digestive cavities penetrating the axis. 
Axogaster, g.n. 
Fixed, base a branched, creeping root; axis calcareous divided into segments 
with small intervals ; articular surfaces of the segments convex or concave ; ab- 
dominal cavities inserted in the segments, progressively obliterated ; crenulations 
in sides of the cavities less than twelve: nature of encrusting layer unknown, 
lateral segments produced on the side of the calcareous portions of the axis. 
Axogaster cretacea, sp.n. (Tab. XVIII. figs. 29 to 33.) 
Segments slender, generally long, unequal in outline, often bent, traversed 
longitudinally by waved ribs or furrows ; articular surfaces grooved concentrically 
with a central boss or pit; intervals between segments very small, occasionally 
represented by an indented line; lower end of lateral segments implanted in 
side of main segment ; abdominal cavities irregularly distributed, round or oyal, 
