The internal structures and development of the genus Holocystis Ed. and Haime, of which only one species, the type, has been recorded, have not previously been described. It is the purpose of this paper to repair the omission. This Aporose genus is interesting in that it exhibits certain characters normally well developed amongst Rugosa and in possessing four principal septa arranged in a cruciform manner within the calice, instead of having its septa merely arranged on the hexameral plan characteristic of normal Aporosa. I am greatly indebted to Dr. Stanley Smith for the loan of certain specimens, and for much helpful criticism and advice, and to Professor S. H. Reynolds for access to, and use of, material in the geological collections of the University of Bristol.