AMONGST THE CORALS OF THE PALAOZOIC PERIOD. 247 
III. RELATIONS BETWEEN THE GROWTH OF DIFFERENT PARTS OF A 
CoMPOUND CORALLUM. 
. Amongst the various circumstances which conspire to regulate the growth 
of the corallum, one of the most important is to be found in the presence or 
absence of a common epitheca. Asa general rule, the existence of a common 
epitheca more or less limits the growth of the colony, and the corallum 
assumes a compact and more or less definite form, as is well seen in the massive 
species of Favosites, im Michelinia, and in certain species of Chetetes and 
Alveolites. On the other hand, the absence of a common envelope is usually 
attended by a more or less lax or spreading mode of growth, as is well seen in 
the fasciculate coralla of Diphyphyllum, Eridophyllum, Syringopora, and the 
compound Cyathophylla, as well as in many other forms. 
Even where a common epitheca is present, however, there are great differ- 
ences as to its form and development. In some cases, as in Favosites 
Gothlandica, Lam., Michelinia convexa, D’Orb., and other species of the same 
genus, Strombodes Murchisoni. Edw. and H., Acervularia luxurians, Eichw., 
and other forms, the corallites radiate from a basal point, and the corallum 
has a more or less pyriform or sub-spherical shape, with a pointed base. The 
epitheca covers the under surface of the base, and envelopes the outer sides of 
the peripheral corallites; but the corallum increases mainly in height, and 
the accompanying increase in diameter is effected by the intercalation of new 
corallites between those already existing.. In other cases, as in most examples 
of Favosites hemispherica, Yandell and Shumard, in Fistulipora Canadensis, 
Billings, Alveolites Goldfussi, Billings, Chetetes petropolitanus, Pander (in its 
typical form), and in many species of Heliolites, the epitheca forms an approxi- 
mately flattened plate, to which the corallites are directed more or less at right 
angles, and upon which they rest by their bases, and not by their sides. In 
these cases, therefore, the corallum usually has the general form of a flattened 
or plano-convex expansion, which may increase laterally to almost any extent 
by means of basal or parietal gemmation. At the same time, the vertical 
erowth of the corallum is often carried on by the interstitial addition of fresh 
corallites, or even by the superposition of additional layers of tubes. 
In still another group of cases, namely, in Favosites turbinata, Billings, and 
in some examples of F. Forbesi, Edw. and Haime, the corallum has more or 
less of a cylindrical or turbinate shape, and the corallites radiate outwards from 
a central line, so as to open on the surface nearly or quite at right angles to 
the axis of the colony, except at its upper portion. The epitheca, which at 
first must be confined to the actual base of the colony, in these cases continues 
to grow, and gradually spreads over the calices of all the inferior corallites. 
Thus it comes to pass that in an adult example of /. turbinata we have an 
