corals must
have differed from modern Actiniidea in mesenterial characters, and may
have only possessed Edwardsian mesenteries, or even have possessed only
four mesenteries, in this respect showing close affinities to the
Stauromedusae. Moreover, there are some modern corals in which the
secondary cycle of mesenteries departs from the Actinian plan. For
example, J.E. Duerden has shown that in _Porites_ the ordinary zooids
possess only six couples of mesenteries arranged on the Actinian plan.
But some zooids grow to a larger size and develop a number of additional
mesenteries, which arise either in the sulcar or the sulcular entocoele,
much in the same manner as in Cerianthus. Bearing this in mind, the
following arrangement may be taken to represent the most recent
knowledge of coral structure:--

GROUP A.

Family I. ZAPHRENTIDAE.--Solitary Palaeozoic corals with an epithecal
wall. Septa numerous, arranged pinnately with regard to four principal
septa. Tabulae present. One or more pits or fossulae present in the
calicle. Typical genera--_Zaphrentis_, Raf. _Amplexus_, M. Edw. and H.
_Streptelasma_, Hall. _Omphyma_, Raf.

Family 2. TURBINOLIDAE.--Solitary, rarely colonial corals, with
radially arranged septa and without tabulae. Typical
genera--_Flabellum_, Lesson. _Turbinolia_, M. Edw. and H.
_Caryophyllia_, Lamarck. _Sphenotrochus_, Moseley, &c.

Family 3. AMPHIASTRAEIDAE.--Mainly colonial, rarely solitary corals,
with radial septa, but bilateral arrangement indicated by persistence
of a main septum. Typical genera--_Amphiastraea_, Etallon.
_Thecidiosmilia_.

Family 4. STYLINIDAE.--Colonial corals allied to the Amphiastraeidae,
but with radially symmetrical septa arranged in cycles. Typical
genera--_Stylina_, Lamarck (Jurassic). _Convexastraea_, D'Orb.
(Jurassic). _Isastraea_, M. Edw. and H.(Jurassic). Ogilvie refers the
modern genus _Galaxea_ to this family.

GROUP B.

Family 5. OCULINIDAE.--Branching or massive aporose corals, the
calices projecting above the level of a compact coenenchyme formed
from the coenosarc which covers the exterior of the corallum. Typical
genera--_Lophohelia_, M. Edw. and H. _Oculina_, M. Edw. and H.

Family 6. POCILLOPORIDAE.--Colonial branching aporose corals, with
small calices sunk in the coenenchyme. Tabulae present, and two larger
septa, an axial and abaxial, are always present, with traces of ten
smaller septa.