Prof. H. A. Nicholson—On the Favositide. 109. 
corallite which may readily be mistaken for mural pores. The struc- 
tures to which I allude present themselves as rounded dark dots 
arranged in lines, and presenting very much the appearance of being 
pores. In reality, these structures—which have sometimes been 
confounded by various observers (including myself) with mural pores 
—are the cut ends of septal spines or thorns, which project into the 
cavity of the corallite, and thus come to be divided in sections across 
the visceral chamber. 
3. Vertical sections coinciding with the walls of the corallites.—As 
above mentioned, most vertical sections of the Favositoid Corals come 
here and there to traverse the walls of the corallites. This is 
inevitable when we reflect that the corallites are usually more or 
less curved, so that it is hardly possible for a vertical section to 
divide any single corallite along its whole length in a single plane. 
Hence, a section which in one place cuts a tube through the centre 
of the visceral chamber may in another place divide the same tube 
along one of its bounding-walls (see Figs. 1 A’ B’ and C’). When- 
ever the plane of a vertical section comes to coincide with the plane 
of the wall of a corallite, the mural pores are necessarily shown 
more or less extensively as round or oval perforations in the wall, 
the latter appearing as a more or less opaque space replacing the 
calcitic infilling of the visceral chamber (Fig. 1 A’). Almost all 
vertical sections of well-preserved examples of Favosites, Pachypora, 
Michelinia, Alveolites, etc., show the appearances here described at 
certain points ; and when this is the case, the presence of mural pores 
can never be doubted. In certain species of Favosites, however, as 
for example in F. aspera, D’Orb., and F. Mullochensis, Nich. and Eth. 
jun., vertical sections may fail to show the mural pores, owing to 
the fact that these openings are placed at the angles of the prismatic 
tubes, instead of along the flat faces of the corallites. In the species 
of Alveolites, also, owing to the position of the pores on the short 
sides of the corallites, vertical sections often fail to show the appear- 
ances just described, since the parts of the wall exposed in such 
sections commonly belong to the wide non-poriferous faces of the 
corallites. 
If an investigation of a coral otherwise similar to the Favositide 
should fail to bring to light any of the phenomena above described 
as indicating in thin sections the presence of mural pores, it may in 
general be safely concluded that such a Coral is ‘‘imperforate.” At 
the same time, it is not safe in all cases to conclude that mural pores 
are wanting merely because their presence has not been detected in 
one or two thin sections. There are plenty of cases where a single 
thin section exhibits no traces of mural pores, but where abundant 
evidence of the presence of these structures can be obtained by an 
investigation of a series of such sections. There are also cases 
where a specimen of a form certainly known to possess mural 
pores may nevertheless fail to show signs of these in thin sections, 
as the result of complete recrystallization or replacement. Such 
cases are, however, rare, and do not affect the general value of the 
characters above pointed out. 
