STRUCTURE OF STROMATOPORID &. 49 
permitting all the structures to be very well seen, either on polished 
surfaces or in transparent slices *. 
In these specimens about three interspaces and two laminz occur 
in the space of a millimetre ; and though neither the laminz nor the 
interspaces are uniform in thickness, the latter are about twice the 
width of the former. In some places the lamin rise into conical 
or rounded eminences with corresponding depressions; in others 
they are nearly flat and concentric, this difference being apparently 
accidental. The lamine are connected with each other by pillars, 
which are either round or somewhat flattened (PI. III. fig. 1). The 
texture of the lamine is not spicular, but perfectly continuous and 
finely granular, as if made up of minute fragments of calcite. When 
the mass is broken parallel to the laminz, the pillars appear as 
minute tubercles (Pl. LV. fig. 5), but a true exterior surface is smooth. 
The laminz are pierced with numerous round pores about one tenth 
of a millimetre in diameter. Some of these pass through hollow 
pillars across one interspace into another; but the greater part pass 
through the lamin from one interspace into the next. The lamin 
themselves are here and there pierced with horizontal tubes which 
thicken the lamin where they pass (Pl. III. fig. 3,6); they appear 
to traverse the laminee obliquely from one space into another, or from 
the hollow pillars laterally. They may be called canals. In addition 
to the ordinary laminw, some of the chambers or interspaces are sub- 
divided by very thin secondary lamine. In a few cases these are 
attached to ordinary laminz as a sort of inner wall. The ordinary 
lamin in the more regular specimens are often of great continuity, 
extending without interruptions for several square inches. 
In some specimens there are a few rounded perforations, less 
than a millimetre in diameter, which extend yertically through 
several interspaces. Their walls are densely calcareous. They are 
often covered up with the growth of the lamin, and seem to have 
no connexion with the other parts of the structure. I do not regard 
them as oscula, but as perforations of some parasitic animal; and I 
attribute to the same origin certain rounded cayities, similarly walled, 
in other parts of the mass. 
The above is an accurate description of the most common type of 
Stromatopora when, as nearly as possible, in its natural condition. 
Other species of the genus, as now usually limited, differ principally 
in the thicknesses and distances of the lamine, in the number and 
size of their perforations, and in their more or less tuberculated sur- 
faces. In some species the pores are so numerous that the laminz 
appear reticulated; in others the lamine are so much thickened 
that the pores appear as tubuli. The pillars also differ somewhat 
in size and form, In the very numerous specimens which I have 
examined, I haye convinced myself that while the lamine are always 
porous they are neyer spicular, and that the so-called oscula are ac- 
cidental. They are due either to the causes above referred to or to 
* These specimens are associated with a beautiful Saccammina, having a cal- 
careous test, which I have described as Saccammina eriana, 
Q.J3.G.8. No. 137. E 
