INTRODUCTION. 9 
All the varieties of length, thickness, porosity, solidity, and ornamentation, observed 
on the septa are represented in the costal structures. As a rule, the cost are shorter 
than the septa in transverse section, but there are many exceptions to it, and it is very 
common to find a rudimentary septum of a high cycle with a corresponding well-developed 
costa.! The projection of the coste from the wall and the size of the space between 
them (intercostal space) vary greatly ; in some species the coste are close and form 
simple prominent ridges, whilst in others they are wide apart, project greatly, and may be 
covered with great spines, dentations, or serrations. The greater projection of certain 
costa, the ornamentation of others, and their correspondence with the cyclical arrange- 
ment of the septa, are readily studied in different species. 
The costze do not always project at right angles from the wall, and those that are 
very long often curve and twist. Whatever may be their form or length, they have sides 
and a free surface. he sides of neighbouring cost are frequently joined by the 
dissepiments of the exotheca, or they may be simply marked by dissepiments which do 
not stretch across the intercostal space.” The sides are often spined or granulated, and 
are even perforated in certain species. The variety in the ornamentation of the different 
cycles of costa in the same individual is very interesting, and its study is of great use 
as a secondary method of specific diagnosis. 
In many compound corals the coste of one corallite ran into and join those of the 
neighbouring corallites,* whilst in others, where the walls are fused,* the cost abort 
altogether. ‘There are many species where the cost are simply rows of granules; in 
others the rows of granules’ become lines of slight elevation, and finally well-developed 
coste. The reverse occurs, and well-developed coste on the outside of a calice often 
become granular or even become aborted on the wall.* 
The exothecal dissepiments extend beyond the cost in some instances, and, as a rule, 
the costz are then feebly developed.” The following are some of the most important 
variations in the structure of costa. They may be absent or rudimentary, and they may 
arise on the corallum at various heights from the base. They may be recognised under 
the following aspects:—Small, large, finely granulated, indistinct, generally indistinct 
inferiorly, prominent, prominent near the calice only, prominent inferiorly, sub-equal, 
equal, alternately large and small. As faint ridges, as striae, moniliform, very thin, per- 
forate, wedge-shaped, flexuous, broad, flat; formed by a series of globules, spines, and 
granules ; wide apart, close, rounded, cristeeform, tubercular, largely spined, dentate, alz- 
form, crenulated, striated, verrucose, folded in zigzag, echinulate, long, dichotomous, 
inclined, &c. 
The cost do not invariably correspond to septa, and are not constantly continuous 
1 Plate IX, fig. 11. 2 Plate IX, fig. 7. 
3 See ‘Descriptions of the “‘ Thamnastree,” in ‘Brit. Foss. Corals, MM. Milne-Edwards and J. 
Haime. 4 Plate IV, fig. 11. 
5 Plate V, fig. 6. 6 Plate I, fig. 4. 7 Plate V, fig. 2. 
