120 ON THE FOSSIL CORALS OF THE 



single septa] fossette on one side ; columella either absent or nidi- 

 mentary. , 



The most simple, or in fact, the primitive type of corals of the 

 order ZoantTiaria Rugosa appears to be a hollow cone, with the inner 

 surface striated vertically by numerous thin elevated ridges, which, 

 commencing at the bottom, ascend in a straight line to the margin 

 of the cup. This type made its appearance during the period ia 

 which the Black Kiver limestone was deposited. I am not aware 

 that any species of the order have ever been found in older rocks. 

 In a beautiful specimen of Fetraia profunda in the collection of the 

 Geological Survey, the conical cup is empty nearly to the bottom, 

 and the vertical ridges or radiating septa, are seen to consist of four 

 groups, each originating in a point at the very apex of the cone. — 

 Three of the divisional lines between these groups are marked by a 

 ridge or septum much stronger than the others. On the exterior of 

 the cup there are three lines exactly corresponding in position to the 

 three large radiating septa within. In all corals of this order belong- 

 ing to the CyathopTiyllidcB that I have seen, where there is a septal 

 fossette, its position is indicated by the central one of the three ex- 

 ternal lines. I have examined a great many specimens of diiferent 

 genera and species from the rocks of this and other countries : and 

 in every instance this rule held good. We can therefore always point 

 out the position of the septal fosette, even when we cannot see the 

 inside of the cup. In Petraia no transverse diaphragms have been 

 observed, but there is unquestionably a septal fossette, which in the 

 bottom is divided into two compartments by the central primary 

 septum. In the genus Zaphrentis we observe another step forward 

 in the complexity of the internal organization. The cone is not only 

 divided into vertical or longitudinal compartments by the radiating 

 septa, but also into horizontal chambers by the transverse diaphragms; 

 these latter being strongly indented in one spot at their margins to 

 form the septal fossette. Out of the various combinations of these 

 four elements — 1st, the epitheca, or thin external wall of the cone ; 

 2nd, the radiating septa, extending from the inside of the epitheca 

 towards the centre ; 3rd, the transverse diaphragms ; and 4th, the 

 septal fossette — many genera have been formed, some of which no 

 doubt must be referred back to Zaphrentis. 



