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per cent) ; of sixteen genera from the Carboniferous seven were 
colonial (forty-five per cent). Froyn these percentages it appears 
that there was a gradual increase in the number of colonial 
genera, but there is no doubt that the majority of the Tetracoralla 
always remained solitary. 
There are few known genera of Hexacoralla from the Paleo- 
zoic, and the greater number of these, including the Archaeo- 
cyathidae and several genera in the Bupsammidae, were solitary. 
The sudden extinction of the Tetracoralla at the close of the 
Paleozoic was followed in the beginning of the Mesozoic by as 
sudden an increase in the number of Hexacoralla. Paleontology 
does not show any definite increase in the proportion of colonial 
to solitary genera among the Hexacoralla ; but so far as is known 
the Hexacoralla have been predominantly colonial from the Tri- 
assic, ninety per cent of the Triassic genera listed by Zittel being 
colonial. 
Hence it appears that in the course of geologic time a predomi- 
nantly colonial type has superseded a predominantly simple type 
of coral. 
Of the five families of Tetracoralla given by Zittel, three 
(Cyathaxonidae, Paleocyclidae, and Zaphrentidae) contain soli- 
tary forms only. In the Cystiphyllidae colonies are rare; hence 
nearly all the compound Tetracoralla are found within the family 
Cyathophyllidae. In this family a definite progression in colony- 
formation may be traced. 
H eliophyllum may serve as an example of a solitary Tetracor- 
allum; this genus also, though rarely, forms dendroid colonies. 
Diphyphyllum forms large colonies, but is composed of dis- 
crete columns held in place by intercolumnar braces placed at 
different levels throughout the colony. Reproduction is by mul-* 
tiple gemmation, and in some species at least the parent polyp 
dies. Since the columns are separated by a space which may be 
as great as their own diameter, and since there is no trace of 
interstitial tissue other than the occasional bridges, it would ap- 
pear that only a few neighboring polyps were continuous over 
newly-formed braces. 
Cyathophyllum represents a further step toward the formation 
of a compact colony. Connecting braces are not present, and a 
fairly distinct progress is traced from species like C. cespitosum 
in which the column walls diverge, through forms such as Cyatho- 
phyllum calvini in 'which the perfect cylindrical column walls be- 
come flattened where two are in contact, to forms like C. hex a- 
