Prof. H. A. Nicholson—Structure of Cleistopora. 151 
This characteristic structure is shown equally well both by vertical 
and tangential sections (Fig. 1, C and D). 
The walls of the corallites are thick, and when viewed in section 
under the microscope exhibit tolerably well the peculiar fibrous and 
subcrystalline structure which is seen in sections of many recent 
Corals. A true “primordial wall,” such as is seen in many species 
of Favosites, does not seem to be present; but the junction between 
adjacent corallites is usually marked by an irregular dark line. The 
visceral cavities of contiguous tubes are placed in communication 
by irregular canals, which represent the “mural pores” of the 
Favositide. Septa are only represented feebly, by a number of 
obscure ridges or striz. 
& 
Fie. 1. Cleistopora geometrica, Edw. & Haime, sp. A. Upper surface of a full- 
sized individual, of the natural size. B. A single calice enlarged. C. Vertical 
section of a specimen growing upon a Brachiopod, enlarged five times. LD. 
Tangential section of the same specimen similarly enlarged. 
From the above description it will be evident that Cleistopora 
geometrica KH. & H.sp.,is a Perforate Madreporarian, and that it 
is related to Protarea, EK. & H. In this latter genus, however, 
the visceral chambers of the corallites are very short, and are not 
filled up in their basal portion with the reticulated endothecal tissue, 
which is characteristic of the genus Clezstopora. 
From the genus Michelinia, De Kon., Cleistopora is sufficiently 
separated, not only by the possession of the peculiar trabecular 
infilling of the lower part of the visceral chambers, but also by the 
total absence of tabula, while the “mural pores” are only repre- 
sented by vermicular and tortuous tubes passing through the walls 
of the corallites. In the general aspect of the corallum, Cleistopora 
geometrica presents a considerable resemblance to the species of 
Pleurodictyum, Golf., from which, however, it is separated by 
characters similar to those which distinguish it from Michelinia. 
