INTRODUCTION. 15 



All the varieties of length, thickness, porosity, solidity, and ornamentation, observed 

 on the septa are represented in the costal structures. As a rule, the costse 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 costse from the wall and the size of the space between 

 them (intercostal space) vary greatly ; in some species the costse 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 

 costse, the ornamentation of others, and their correspondence with the cyclical arrange- 

 ment of the septa, are readily studied in different species. 



The costae 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. The sides of neighbouring costse 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 costse 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 costae of one corallite run into and join those of the 

 neighbouring corallites,^ whilst in others, where the walls are fused,* the costse abort 

 altogether. There are many species where the costse are simply rows of granules ; in 

 others the rows of granules^ become lines of slight elevation, and finally well-developed 

 costse. The reverse occurs, and well-developed costse on the outside of a calice often 

 become granular or even become aborted on the wall.^ 



The exothecal dissepiments extend beyond the costse in some instances, and, as a rule, 

 the costse are then feebly developed.''' The following are some of the most important 

 variations in the structure of costse. 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 strise, moniliform, very thin, per- 

 forate, wedge-shaped, flexuous, broad, flat ; formed by a series of globules, spines, and 

 granules ; wide apart, close, rounded, cristseform, tubercular, largely spined, dentate, alse- 

 form, crenulated, striated, verrucose; folded in zigzag, echinulate, long, dichotomous, 

 inclined, &c. 



The costse do not invariably correspond to septa, and are not constantly continuous 



1 Plate IX, fig. 11. 2 Plate IX, fig. 7. 



8 See ' Descriptions of the " Thamnastreea," in 'Brit. Foss. Corals,' MM. Milne-Edwards and J. 

 Haime. * Plate IV, fig. 11. 



5 Plate V, fig. 6. 6 Plate I, fig. 4. 7 Plate V, fig. 2. 



