(AT 
January 21, 1897] 
It is the structure characteristic of the Madreporarian | 
tissues. 
The endless variety of form, which we find among the 
skeleton. 
skeletal parts, is produced ‘by corre sponding varieties in the 
With this difference, 
calicoblastic layers of the polyp. pa the 
Fic. 1.—Galavea dissepiment: ¢.l., growth-lamelle. 
calcareous lamella is, a 
flesh. 
The cell-for-cell equivalence of the skeleton with the outer 
polypal layers explains why the fine microscopy of fossil and 
s it were, the ‘‘cast’’ of the ectodermal 
Fic. 2.—Calcareous scales on the surface of a dissepiment (from the ccenen- 
chyme of Galaxea) highly magnified. 
WRE 
recent skeletons may guide the systematist in tracing the affini- | 
ties between Madreporarian genera and families. I shall next | 
consider the outstanding varieties of form which I recognise in | 
the septa of typical Madreporaria. 
Septal Forms. 
Bearing in mind the origin of the radial or septal fold as an | 
invagination of the embryonic ‘basal plate” of the polyp, it | 
will ‘readily be understood that there are two opposite calico- 
blastic surfaces in the radial invagination instead of one surface, 
as in the tabula or dissepiment. Fibrous lamelle are formed 
along the entire external surface of the fold. If the invagina- | 
tion is conical, or nearly so, naturally so is the skeletal deposit | 
which it lays down ; if smooth, the deposit is smooth. Such 
is the deposit of the ‘‘ sep/a/ spzne,”’ which we see projecting 
inwards from the wall in many Paleozoic corals, and may still 
find in several species otf Madrepora and other living corals 
The lamelle are formed in. the septal spine around a central 
axis, and as the fibro-crystals are oriented rectangularly in the 
lamella, they radiate out from this axis. 
Considerable notice has been taken in current literature of 
the central part of the spine and of its analogue in the flat 
septum, called the ‘‘ dark line” or ‘‘ primary septum.” I find 
that the opacity of the ‘‘dark line” or ‘‘axis” is due to the 
same cause as the ‘‘dark bands,” or closely-strewn ‘‘ dark 
points,” which occur in the fibro-crystalline deposit in the 
tabula, dissepiment, or any skeletal part. The organic cell 
remnants are, however, massed together in the axial portion of | 
a conical or oblong fold, and the ‘‘dark points” therefore 
appear more prominently in sections of septal spines and septa 
than in sections of a one-sided structure like the dissepiment. | 
There is a further observation. Several of the first few layers 
laid down by the adjacent surfaces of a septal fold are in many 
cases less completely calcified than the next in age. Apparently | 
skeletal deposit accumulates more rapidly at the septal edges | 
NO. 1421, VOL. 55] 
281 
than lower down on the septal sides, but the crystalline changes 
in the cell are less complete. While I regard the presence of 
disintegrating carbon products as the origin il cause of the ** dark 
line,” it is well known that the “‘line”’ may ultimately assume 
various appearances due to secondary changes (see Hinde, “On 
Septastraea,”” Q. J. G. S., 1888), or may be represented by a 
hollow space. 
If the septal invagination is long instead of round in shape, 
it stretches through a certain radial length in the calyx, and 
the calcareous deposit takes the form of a flatizsh septal plate 
in consequence. The lamella are symmetrical on either side 
of the ‘‘ dark line,” which indicates the axis or median plane 
of the fold. We are familiar with microscopic transverse sec- 
tions of flat, plano-symmetric septa in Paleeozoic Zaphrentids, 
as well in our own Turbinolids. The septal spine and the 
plano-symmetric septum are the two most primitive forms of 
septal deposit, and sometimes the flat septum passes at its inner 
edge into spinate prolongations. 
We now come to more elaborate forms. 
as 
The strzated septum 
(Fig. 3) develops within the flaps of a septal fold, which, in- 
Fic. 3.—Striated septum of Galaxea. The stria diverge fan-like from an 
*“area of divergence” (a.«/.) 3 g.« growth- curve. 
stead of being smooth, is thrown into a regular system of pleats, 
and has a goflered edge. The surfaces of the calcareous Jamellz 
are consequently marked by strice (str.) and grooves, and the 
striz taper to fine serra (sr.) at the edge of the septum. In 
microscopic transverse sections of striated septa, the fibro- 
crystals radiate out from ‘* dark points, ” better called ‘‘ centres 
of calcification,” which form a row in the median plane (Fig. 4). 
Each ‘“‘centre” represents in cross-section the long axis of a 
stria, or pair of stria, according as the stricze are alternate or 
opposite on the two surfaces of the septum.» A number of 
wave-units of the lamellze are arranged around each axis, hence 
the unit-bunches of fibres, though minute in themselves, com- 
bine to form a relatively large, radiating bunch to which I have 
given the name of ‘* Sasct It passes oblicuely upwards and 
outwards through the series of manele and gives rise to a slight 
eminence or ‘‘granulation” where it emerges at the surface. 
Striated septa occur in Stylinidze, Oculinide, some Turbinolide, 
&c. ; they are composed entirely of fascicles bisymmetrically 
arranged on either side of median axes. 
It is a further step in complication of structure to pass from 
the striated septum to the roughly-granulate, ridged, spiniform- 
toothed septum which one sees in many Astreids, é.2. Mussa. 
Each broad ridge that passes downwards from a single spiniform 
