* 
AMONGST THE CORALS OF THE PALAOZOIC PERIOD. 241 
coral as the Lower Silurian Protarea vetusta. In forms such as the above we 
have basal gemmation in its purest form, constituting the only method of 
increase ; and in all such the colony increases in dimensions solely by the 
addition of new corallites at the margins or circumference of the mass. There 
are, however, many cases in which increase is effected by a combination of basal 
gemmation with some other mode of growth. Thus, in all the typical species 
of the genus Syringopora, the colony extends itself laterally by marginal bud- 
ding, whilst it is at the same time increasing vertically by means of parietal 
gemmation. The same is also true, though to a more limited extent, in the 
Tubulose genus Awlopora. In most, if not all, of the massive species of Favo- 
sites, again, in such massive forms of Chetetes, as C. petropolitanus, Pander, 
in the massive species of Heliolites and its allies, in the genus Fistulipora, and 
in many other Palzozoic corals, there is a combination of basal gemmation, with 
a peculiar form of lateral gemmation, or with fission. Basal gemmation, pure 
and simple, is therefore to be regarded as of comparatively rare occurrence 
amongst the ancient corals, and perhaps is only seen in those forms which con- 
stitute very thin crusts, the corallites of which are all of nearly equal height, and 
which do not tend to increase vertically. 
D. Parietal or Lateral Gemmation.—Parietal or lateral gemmation is one of 
the commonest and best known of all the modes of increase of the coralligenous 
Actinozoa ; and it consists in the production by the parent corallite of a bud at 
some point in its walls, between the lip of the calice and the base. Sometimes 
the primitive corallite may not produce more than one or two of these lateral’ 
buds. More commonly the secondary buds repeat the process of gemmation, 
and in a very great number of instances there is a combination of basal with 
lateral budding ; so that whilst the corallum is increasing in height by virtue of 
the latter process, its lateral limits are simultaneously extended by the former. 
Excellent examples of parietal gemmation amongst the Paleozoic corals may 
be found in the genus Diphyphyllum, in some of the species of Erzdophyllum, in 
Lithostrotion and Syringopora, and in certain species of Cyathophyllum, Cysti- 
phyllum, Pachyphyllum, and Heliophyllum. The general form of corallum pro- 
duced by means of parietal gemmation varies greatly in different cases. The 
typical form, so far as regards the Paleozoic corals, may be regarded as being 
the more or less loosely fasciculate corallum of Diphyphyllum arundinaceum, 
Billings, D. Archiaci, Billings, Eridophyllum Vernewilanum, Edw. and H., 
Cystiphyllum fruticosum, Nich., the ordinary form of Cyathophyllum ceespitosum, 
Goldfuss, Lithostrotion irregulare, Phill., and many other species which could be 
mentioned. In these cases the corallites are more or less elongated and cylin- 
drical in form, and are not actually amalgamated with one another, being, on the 
contrary, usually separated by slight but conspicuous intervals; the buds are 
produced at compartively long intervals, and the parent corallites continue to 
VOL. XXVII. PART IIl. 3R 
