This remarkable foraminifer, which contributes so largely to the formation of some of our Carboniferous Limestone beds, was first described by H. B. Brady in 1871, and assigned by him to the existing arenaceous genus Saccammina (Sars). His views on its structure and taxonomic affinities have been accepted without question by all subsequent writers, and are embodied in our text-books on palæontology. Brady himself, however, was evidently puzzled by certain features presented by the structure of the shell: thus, in his latest description, n3 after stating that the shell is compact and arenaceous with a nearly smooth exterior, he remarks (p. 60) of the interior surface that it ‘varies a good deal in different specimens,’ sometimes being ‘smooth or roughened only by the projecting angles of the constituent sand-grains [and at others] covered with a network of short delicate labyrinthic growths.’ Of the minute structure of the shell he finds it ‘difficult to speak in positive terms, owing to peculiar conditions of infiltration.’ Finally, he calls attention to certain ‘minute circular scars, too frequent and uniform not to have a meaning, [and suggests] … that they may result from the repair of an injury to the test.’ As a specimen of the Saccammina Limestone, from the classic locality of Elfhills in Northumberland, crowded with remains of this organism, forms part of the teaching collection in the University of Oxford, it devolved upon me to make myself familiar with its structure. My examination led me to recognize the fidelity of Brady's