242 PROFESSOR NICHOLSON ON THE MODE OF GROWTH AND INCREASE 
grow without being injuriously affected by the production of their buds ; whilst 
all the stems preserve an approximate parallelism with one another, or only 
diverge slightly in proceeding from the base. 
In other cases, as in Heliophyllum proliferum, Nich. and H., subceespitosum, 
Nich, the principle of growth is the same as in the preceding, but the parent 
corallite does not produce more than one, two, or three lateral buds. 
In other cases, as in Lithostrotion basultiforme, the method of growth is 
the same, but all the corallites are in close contact; and in others they 
become actually amalgamated by their walls, giving rise to a regular astreeiform 
corallum. 
As has been before mentioned, parietal budding may be combined with 
other modes of growth, when its existence is sometimes difficult to demonstrate. 
Thus, it may be combined with calicular gemmation (Hridophyllum strictum, 
&c.), or with basal gemmation (most of the species of Favosites, Cheetetes, 
and Syringopora). These complex methods of growth may give rise to simply 
fasciculate coralla, as in the species of Hrzdophyllum just alluded to; but they 
more commonly result in the production of a massive corallum. This latter 
result is well exhibited by forms like Favosites Gothlandica, Lam., F. hemis- 
pherica, Yand. and Shumard, and Chetetes (Monticulipora) petropolitanus, Pander. 
Tn these and similar cases, the corallum is usually of a more or less hemispheric 
or subspherical, or pyriform shape, and is composed of corallites, which diverge 
more or less from an imaginary axis, a basal point, or a basal plane. The 
corallites are closely contiguous, but are not amalgamated by their walls; and 
the increase of the corallum is effected partly by the production of buds round 
the margin of the mass, and partly by the interpolation of new corallites, 
produced by lateral gemmation from the sides of the old tubes. As will be 
noted however, subsequently, it is often extremely difficult to decide in certain 
of these cases whether the growth has been produced by fission, or by lateral 
gemmation. 
E. Increase by Fission—Fissiparous growth amongst the corals is effected 
by the cleavage or division of the calice, so that two corallites are produced out 
of one. This process is so well known, that it will not be necessary for me to 
enter into any details about it, further than to notice certain of its modifica- 
tions, as exhibited amongst the Palzeozoic corals. The chief practical point to 
be observed is, that there is great difficulty, in many instances, in distinguish- 
ing between corals formed by fission and those produced by parietal gemmation 
on the one hand, and calicular gemmation on the other hand. In many cases 
the distinction is, of course, an easy matter; but in others the results are ex- 
tremely similar, though the processes by which they are brought about are very 
different. Some of these resemblances I shall discuss at a later stage ; in the 
meanwhile, it is sufficient to point out the general reason of their occurrence. 
o 
