t
that they show a tetrameral instead of a hexameral symmetry. Thus in the
family _Stauridae_ there are four chief septa whose inner ends unite in
the middle of the calicle to form a false columella, and in the
_Zaphrentidae_ there are many instances of an arrangement, such as that
depicted in fig. 19, which represents the septal arrangement of
_Streptelasma corniculum_ from the lower Silurian. In this coral the
calicle is divided into quadrants by four principal septa, the _main
septum, counter septum_, and two _alar septa_. The remaining septa are
so disposed that in the quadrants abutting on the chief septum they
converge towards that septum, whilst in the other quadrants they
converge towards the alar septa. The secondary septa show a regular
gradation in size, and, assuming that the smallest were the most
recently formed, it will be noticed that in the chief quadrants the
youngest septa lie nearest to the main septum; in the other quadrants
the youngest septa lie nearest to the alar septa. This arrangement,
however, is by no means characteristic even of the Zaphrentidae, and in
the family _Cyathophyllidae_ most of the genera exhibit a radial
symmetry in which no trace of the bilateral arrangement described above
is recognizable, and indeed in the genus _Cyathophyllum_ itself a radial
arrangement is the rule. The connexion between the Cyathophyllidae and
modern Astraeidae is shown by _Moseleya latistellata_, a living
reef-building coral from Torres Strait. The general structure of this
coral leaves no doubt that it is closely allied to the Astraeidae, but
in the young calicles a tetrameral symmetry is indicated by the presence
of four large septa placed at right angles to one another. Again, in the
family _Amphiastraeidae_ there is commonly a single septum much larger
than the rest, and it has been shown that in the young calicles, e.g. of
_Thecidiosmilia_, two septa, corresponding to the main- and
counter-septa of Streptelasma, are first formed, then two alar septa,
and afterwards the remaining septa, the latter taking on a generally
radial arrangement, though the original bilaterality is marked by the
preponderance of the main septum. As the microscopic character of the
corallum of these extinct forms agrees with that of recent corals, it
may be assumed that the anatomy of the soft parts also was similar, and
the tetrameral arrangement, when present, may obviously be referred to a
stage when only the first two pair