246 PROFESSOR NICHOLSON ON THE MODE OF GROWTH AND INCREASE 
and C. Steiningeri, Edw. and H., growth is by lateral gemmation; whilst dicho- 
tomous fission is stated to occur in some examples of C. cwspitosum, Gold. 
Examples very similar to the above could be drawn from other genera, and, 
indeed, the same fact is proved by the common co-existence in the same genus 
of simple and compound species. Some genera, such as Zaphrentis, Aulaco- 
phyllum, Lophophyllum, and the like, are so far only known as containing simple 
corals. In fact, the existence of a simple corallum is usually given as one of 
the characteristics of the entire family of the Zaphrentinw. Even in this family, 
however, there are genera in which future researches will probably place certain 
compound forms. The genus Amplerus, for example, is usually stated to con- 
_tain only simple coralla. There exists, however, in the Niagara and Guelph 
formations (Upper Silurian) of North America, and in the Corniferous Lime- 
stone (Devonian) of the same country, a coral which has all the structural 
peculiarities appertaining to Amplexus, but is truly compound, and grows in 
fasciculate masses. One course, in this and similar cases, would be to affirm 
that, being composite, it could not belong to the same generic group; but it 
appears to be more philosophical to alter the received definition of the genus, 
and to admit into it both compound and simple forms. Other instances in 
which the same genus contains both simple and compound species are so fre- 
quent as not to require special mention. It only need be added in this connec- 
tion that there are not infrequent instances in which both simple and compound 
individuals are found in the limits of the same species. As examples of this 
may be mentioned Heliophyllum Halli, Edw. and H., H. subccespitosum, Nich., 
and Diphyphyllum Archiaci, Billings. 
d. Finally, it may be pointed out that precisely the same mode of growth, 
even when this is very peculiar, is exhibited by corals belonging to the most 
diverse groups. The best example of this that can be given is the existence of 
the same kind of calicular gemmation in such distinct and remote genera as 
Paleocyclus and Cyathophyllum, the one belonging to the Aporose division of 
Zoantharia sclerodermata, the other being one of the Rugosa. 
e. The general conclusion from the above facts would seem to be that too 
much stress must not be laid upon the mode of increase of the corallum as a 
means of classification. When accompanied by other well-marked peculiarities 
it has undoubtedly a classificatory value; but not otherwise. . Allied forms in 
the same genus often exhibit very different modes of growth; the same is true 
in some instances even of the individuals of a single species, whilst forms 
belonging te the most diverse groups may increase in the same way. Finally, 
there is often such great difficulty in determining what is the true mode of 
erowth amongst the fossil corals, that this character, even if theoretically im- 
portant, loses its value altogether when employed practically as a means of 
separating different genera. 

