Rev. T. Hincks on the Genus Retepora. 361 
it 1s almost entirely concealed by the secondary orifice. In 
this we find the characteristic orbicular shape, the large loop- 
like sinus, and the marginal processes and avicularium. The 
remarkable spine (Pl. XIX. fig. 3) which rises on one side 
of the orifice seems to have escaped observation. Its struc- 
ture is peculiar. At the base it is articulated by a corneous 
joint to a small tubular process on the margin, and is com- 
posed of a number of segments or pieces, each of which is 
contracted below and expands upward, and seems to fit into 
the one beneath it; so that the spine presents an uneven out- 
line and has the appearance of being jointed at pretty regular 
intervals. The large aviculiferous processes are often present 
in profusion and give a very marked character to the zoarium ; 
occasionally, however, they are scantily developed. The 
variety of the avicularian appendages in this species is re- 
markable. In addition to those which have been described, 
there is occasionally a gigantic avicularium, exceeding the 
zoocecium in length, which occupies an elevated space at the 
top of the fenestre, with a much elongated subspatulate man- 
dible directed obliquely downwards. ‘ihis is sometimes re- 
placed by a very curious form, which I have not met with 
elsewhere. It is narrow-elliptical in shape, usually large, 
with a very solid semielliptical mandible, of a dark horn- 
colour (Pl. XIX. fig. 5). This form, I believe, is really an 
aborted condition of the gigantic avicularium just described, 
and consists essentially of the basal portion of the latter minus 
the long mandible. 1 was at one time induced to think that 
the form with the gigantic avicularia should be accounted 
distinct; but in its minute characters it agrees with the 
present species. It is perhaps worthy of being distinguished 
as 2. monilifera, var. munita. 
lt should be noted that the aspect of the cell, and especially 
of its oral aperture, is subject to great variations, corre- 
sponding with the stages of growth and development. After 
the formation of the secondary orifice has been commenced, 
its sinus appears as a very small slit almost closed above, 
and the avicularium lies transversely on the margin beside it, 
without being elevated, as it is subsequently, on a mucronate 
process. 
The peculiar structure of the ovicell, which has suggested 
the specific name, seems to be due to the filling-in of the fissure, 
which exists in the usual condition on the younger cells, with 
a granulated calcareous plate. ‘The oocecia are commonly 
developed in such quantities as almost to conceal the surface 
of the zoarium ; and this profusion may perhaps be accounted 
a specific character. 
