168 PROF. P. M. DUNCAN ON STREPTELASMA REMERI. 
direction. No coste pass from these vertical ones, but they com- 
mence at the calicular margin and pass downwards and outwards, 
impinging, one after the other, on the edges of the longest and nearly 
vertical and outermost coste of the sets which are pinnate on the 
vertical pair on either side (fig. 6). The most oblique coste 
of the fourth set soon reach those against which they impinge and 
are short ; and with increasing length the rest of the costz diminish 
in the obliquity of their course, and become at last nearly parallel 
with those which are along the median line on the concave side. 
Hence the costee of the fourth set are semipinnate on the lhmiting 
pinnate costa of the second and third sets on either side of the 
corallum at the quadrants of the margin. This double pinnation 
produces a very remarkable ornamentation, which, although a few 
more coste may be introduced, is invariable. 
The coste may have a plain outer surface, or they may be covered 
here and there, with a close cross-ridging resembling a very delicate 
epitheca, but it is inseparable from the costal structures (fig. 6). 
In a few instances the coste are rather more prominent than in the 
type. 
Finally, as regards the outside of the corallum, it is marked 
more or less by feeble growth-swellings and transverse contractions 
(fig. 4). 
Seen in the calice the septa are distinct, stout, curved at their 
free upper margin, often bilobed, and variable in their length and 
direction around the fossa and fossula. Those which reach far in- 
wards and bound or enter the central or axial fossa are about 
twenty-five in number; and there is one (and sometimes there are 
two) longer than the others. A variable number of small septa 
exist, and they are placed between all or only a few of the larger. 
The fossula may or may not correspond with the vertical pair of 
costee: and when it is fairly developed the long septum is usually 
opposite (fig. 1); or there may be no fossula, and in its stead two 
long opposite septa. When the fossula is well developed it may be 
bounded by well-developed septa, and may contain one, or even 
two, smaller septa (fig. 7), a long one being sometimes between 
them. The direction of the septa on the calice is not universally 
radial, for in one half—and that which is remote from the vertical 
pair of coste—a pinnate arrangement in relation to a large septum 
is very constant. No union of the ends of the septa is visible at the 
calice ; they do not transgress upon the surface of the fossa; and 
they may be flat, concave, and marked with a slight projection. 
Two conditions of growth of the septa and cost are noticed: in 
one the septa are continuous with the centre of the respective coste ; 
and in the other the intercostal space fits into the median line of 
the septum (fig. 8). When transverse sections are made with the 
view of studying the septa, very different appearances are presented 
from those observed in the calices; and the sections made just below 
the calice and midway down, differ remarkably in their illustration 
of the septal arrangement. 
The principal differences are, that in the sections made below the 
