Specific characters.—‘Upper’ dental plate narrow, triangular in shape, and scarcely curved; antero-lateral and posterolateral nearly straight, the inner border bulging downwards in its anterior half. Antero-lateral half of the coronal face raised into a prominent round ridge, which is partly subdivided by transverse constrictions into a few large rounded bosses; postero-lateral half flattened, curving a little upwards at the postero-lateral border. Remarks.—This species is known only by two satisfactorily preserved ‘upper’ dental plates and a few fragments. The type-specimen (B.M. P10335) measures 12 mm. in width at its inner border, while the second specimen is somewhat larger. In the former five distinct bosses are seen on the rounded antero-lateral ridge, and there would probably be a sixth at the apex, which is broken away. In both cases the coronal face is much abraded, so that the punctations of the dentine, below the original surface, are conspicuous; but in the larger specimen there are still traces of sigmoidally-curved lines extending from the grooves between the antero-lateral border: these suggest that the dental plate results from the fusion of Helodus -shaped teeth in which the coronal eminence occupied the anterior half. D. garwoodi , which is named after its discoverer, differs from all other known species of the genus in the coarse heading of the antero-lateral eminence of its dental plate, the relative evenness of its postero-lateral half, and the slightness of its curvature. A fragment from the St. Louis Limestone of Illinois, which has been referred to D. intermedius , perhaps approaches it most