The skull of one species (A. Zitteli) of this remarkable ungulate has already been figured by Mr. Beadnell, and also in this Magazine (December, 1903), where its general form is well shown. Details of the structure of the skull and skeleton will be given in the monograph, so that only a few of the more important characters need be referred to here.

The pedunculate occipital condyles are very large and prominent; the occipital surface slopes strongly forwards and is bordered by a massive lambdoidal ridge, which on either side (in old animals at least) rises into a prominent backwardly directed boss of bone, almost like a blunt horn. The parietal region of the cranial roof is flat and is at right angles to the side walls of the skull, being sharply marked off from them by well-defined ridges, which form the upper limits of the temporal fossæ. The suture between the parietals is obliterated in the youngest skull examined. The pair of small posterior horns over the orbits are borne exclusively by the frontals, while the great anterior pair seem to be formed entirely by the enormously developed nasals.