74 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
with thin walls and a general vertical ascending course. The colony 
being flat and explanate, this central cord may be described as an axial 
plane consisting of hollow tubes arranged four deep, but extending right 
through the frond, except at its extreme lateral edges. As these central 
tubes grow upwards new tubes are constantly being formed, and inserted 
amongst them. Thus in the course of growth the number of tubes is 
constantly increasing, and in order that there may be room for the in- 
creasing number, some of the tubes are pushed on one side, and that at 
such a sharp angle, that some come to lie at right angles to the ascending 
central tubes ; the external ends of the tubes, after they have been 
deflected outwards, become thickened by a secondary deposit of carbo- 
nate of lime within their cavities ; among them the calicles are formed, 
which are likewise set nearly perpendicularly to the central cord. A 
c/Lsideration of the mode of growth of Heliopora explains several 
features in the corallum. After the tubes have been pushed away from 
the vertical and have assumed a more or less horizontal position, they 
are closely packed together and there is no room left for the intercalation 
of new tubes. Growth consequently ceases in the lateral parts of the 
colony, and the activity of the calycoblasts is limited to the formation 
of secondary deposits within the cavities of the tubes, and to the formation 
of the characteristic echinulations at the points where the walls of 
adjacent tubes unite. It is urged that a close study of the structure and 
mode of formation of the hard parts of Heliopora is of importance, as 
extinct corals can bo judged only by a study of their hard parts. After 
pointing out some errors in Prof. Nicholson’s descriptions, who assumes 
that, wherever the walls of the corallites or coenenchymal tubes of an 
extinct coral are not obviously separated from one another, they must 
nevertheless have been originally distinct and have been subsequently 
fused together, Mr. Bourne suggests that tabulate Alcyonarian corals 
should be divided into two distinct groups according to the characters of 
their skeletons. In the one, each corallite, and if they are present, 
each coenenchymal tube, has no distinct and proper wall, and if the 
corallites are closely apposed so that their walls become contiguous this 
primitive distinction is not lost. This group is represented to-day by 
Tubipora, and it is proposed to call it the Autothecalia. In the second 
group the walls of the calicles and coenenchymal tubes are not separate 
and independent, but the pieces forming the wall of each take a share in 
forming the walls of contiguous tubes or calicles. This group, repre- 
sented to-day by Heliopora , receives the name of Ccenothecalia. To 
Tubipora is allied the extinct Syringopora. It is exceedingly probable 
that Syringo'lites and the Favositidse belong to this group, while the 
Columnariidse may be provisionally placed in it. Among extinct Coeno* 
thecalia are Heliolites , Plasmopora, and Propora. The Chsetetidae must 
certainly be ranked among them in spite of the absence of coenenchymal 
tubes, and the Monticuliporidae may be provisionally placed in the same 
group, though perhaps they will hereafter be shown to be Autothecalia. 
A chance examination of some members of the Xeniidae has shown 
that the spicular skeleton of Alcyonaria, other than Heliopt ra, may als'* 
be ectodermic. A new species of Xenia , X. garciee , is described, and 
reasons are given for retaining the genus Heteroxenia. X. umbellata , 
X. garcise , and II. elizabethse display successive grades of differentiation 
