108 Prof. H. A. Nicholson—On the Favositide. 
Alveolites itself, the mural pores are generally uniserial, and are 
placed along the short sides of the compressed corallites ; so that they 
appear in tangential sections as gaps at the ends of the crescentic 
tubes (Fig. 1, B). It is hardly necessary to add that a tangential 
section is very unlikely to traverse more than an occasional mural 
pore. It is, therefore, only here and there in such a section that we 
should find the deficiencies in the walls of the corallites due to the 
presence of mural pores; and in some sections we might fail to find 
any traces of the pores. 
The only appearance which, so far as I know, could be confounded 
with the above is the apparent communication between two adjacent 
corallites seen in tubes of Chetetes, and of certain other types, as viewed 
in tangential sections. In these latter cases, however, it is not that 
two adjoining corallites are placed in communication by a perforation 
in their wall, but that one corallite is imperfectly separated into two 
by the extension into the visceral chamber of a septiform plate, the 
result of commencing fission. It is hardly possible for a practised 
observer to confound the appearances produced in this way with those 
due to the presence of mural pores; and in any case an examination 
of longitudinal sections would at once show the true nature of the 
phenomenon. 
2. Vertical sections traversing the visceral chambers of the corallites. 
—Vertical sections of the Favositoid Corals—i.e. sections cutting the 
tubes longitudinally—usually in part traverse the visceral chambers 
of the corallites, while in part they coincide more or less closely 
with the walls of the corallites. In the former case, the section 
exhibits the infilling of the tube (calcite or mud), intersected by the 
tabulee and bounded laterally by the longitudinally divided wall of 
the corallite on each side (Fig. 1, 4’, B’, and C’). In such cases, the 
presence of mural pores can commonly be recognized by the more or 
less frequent occurrence of gaps or deficiencies in the wall on each 
side, the cavity of the tube being thus placed in communication with 
adjoining visceral chambers. The phenomena presented in these 
cases are precisely similar to those shown in tangential sections, and 
need no special notice. As in the latter, it is only here and there 
that the line of section happens to coincide with a mural pore, and 
it is, therefore, only here and there that we observe in long sections 
the gaps in the bounding-walls of the corallites caused by the presence 
of pores, the size and number of these gaps depending directly upon 
the size and number of the pores. As a rule, however, the presence 
of mural pores can in this way be readily recognized in thin vertical 
sections of Favosites, Michelinia, Pachypora, Alveolites, and the other 
Favositoid Corals (see Fig. 1). 
For obvious reasons, longitudinal sections which more or less 
closely coincide with the centre of the visceral chamber of a corallite 
cannot exhibit any signs of mural pores other than the gaps just 
mentioned in the bounding-walls of the tube. In other words, no 
trace of the mural pores can be found in the calcite or mud which 
occupied the space between the bounding-walls. It is not uncommon, 
however, to find certain structures in the calcitic infilling of the 
