








240 PROFESSOR NICHOLSON ON THE MODE OF GROWTH AND INCREASE 
becomes truly astreeiform, as is seen in such a species as Cyathophyllum regium, 
Phill., and in many other instances. It is to be remembered, however, that 
many of the astreeiform corals, though often adduced as examples of calicular 
gemmation, really owe their form to a process which would be more appropriately 
termed “ calicular fission.” 
In other cases,compound calicular gemmation gives rise to fasciculate corals, 
as is well seen in forms such as Cyathophyllum articulatum, Wahl., and Erido- 
phyllum strictum, Edw. and Haime. This peculiarity is partly due to the fact, 
that the secondary corallites remain cylindrical, without expanding materially ; 
that they rise side by side without uniting, and without greatly diverging ; and 
that their vertical growth is continued for a considerable period before they in 
turn give rise to calicine buds. Another cause for their fasciculate form is, 
however, to be found in the fact, that in these cases true parietal gemmation is 
often combined with true calicinal budding. There need, therefore, be no sur- 
prise at the assumption by such coralla of the fasciculate form, which is so 
commonly found associated with parietal gemmation. 
Finally, there is a peculiar modification of compound calicular gemmation, 
which I have observed to occur in certain examples of Heliophyllum Hall. In 
this modification, the calice of the primitive corallite throws up a number of 
buds ; but the former does not seem to feel the stimulus to gemmation till it 
has reached a great age, and a corresponding size. The result of this is, that 
the secondary corallites remain more or less aborted, and do not appear to have 
sufficient energy to repeat the process of budding. Hence, in these cases, we 
have the fully-developed and often comparatively gigantic parent corallite, sur- 
mounted by a tuft of small undeveloped secondary corallites, springing from 
different points in its calicine surface. 
C. Basal Gemmation.—This mode of increase, better known as “ marginal” 
or “circumferential” gemmation, is effected by the extension from the base or 
margin of the original polype of a portion of its substance, which throws up a 
bud, and is ultimately converted into a new polype. Very different forms are 
produced in this way, according as the new buds are formed along definite lines 
or stolons, or are developed in a regular manner all round the circumference of 
the primitive mass. It is hardly requisite to add, that in the case of all com- 
pound coralla produced in this manner, the youngest corallites occupy the margin 
of the colony, whilst the oldest are placed in its centre. 
So far as the Paleozoic corals are concerned, marginal gemmation is perhaps 
most perfectly and unmixedly exhibited by those forms which constitute thin 
crusts, parasitic upon foreign bodies, and composed of exceedingly short coral- 
lites. This is the case, for instance, with some of the encrusting species of 
Cheetetes, such as C. Ortont, Nich. (Lower Silurian), and C. quadrangularis, 
Nich. (Devonian). It is likewise the case with such a totally diverse type of 
