In _Hippopotamus_ the skull though essentially like that of the pig is much modified in detail. The brain cavity is very small, while the jaws are immensely developed. The face contracts in front of the orbits and then expands again greatly, to lodge the enormous incisor and canine teeth. The postorbital bar is complete or nearly so, and the orbits project curiously outwards and slightly upwards; the lachrymal is thin and much dilated. The squamosal is drawn out into a postglenoid process, and the hamular process of the pterygoid is prominent. The tympanic bulla is filled with cancellous bone. The mandible is enormously large, the symphysis is long, the angle much expanded and drawn out into a process which projects outwards and forwards. Among extinct forms related to the Suina, _Cyclopidius_ is noticeable for having large vacuities in the lachrymo-nasal region, while _Cotylops_ has the postorbital bar complete; both these forms are from the North American Miocene. In the Tylopoda and Tragulina the skull resembles in most respects that of the Ruminants, shortly to be described; but it is allied to that of the Suina in having the tympanic bulla filled with cancellous bone. The tympanic bulla is better developed in the Tragulina than in most Ungulates. Among Ruminants, the Bovidae, that large group including the Oxen, Sheep, and Antelopes, as a rule have the face bent on the basicranial axis much as in the Suina. The parietals are generally small and early coalesce, the frontals are large and are usually drawn out into horn cores, which are however absent in the skulls of some domestic varieties of sheep and oxen, and also in some of the earlier extinct forms of Bovidae. These horn cores are formed internally of cancellous bone, and on them the true epidermal horns are borne. In young animals there is a distinct interparietal, but this early fuses with the supra-occipital, and in the oxen also with the parietals. The occipital crest is generally well marked, but in the genus _Bos_ becomes merged in a very prominent straight ridge running between the two horn cores; this ridge, which contains air cells communicating with those in the horn cores, is not nearly so well marked in _Bison_. There is often, as in _Gazella_, a vacuity on the side of the face between the nasal, frontal, lachrymal, and maxillae, but this is not found in oxen or sheep. The premaxillae are small, the nasals are long and pointed, and the turbinals are much developed. The Saiga antelope has a curiously specialised skull; the nasals are absent or have coalesced with the frontals and the anterior nares are enormously large. In all Ruminants the lachrymal is large and forms a considerable part of the side of the face; it often bears a considerable depression, the _suborbital_ or _lachrymal fossa_, well seen in most of the smaller antelopes. The postorbital bar is complete, and the orbit is prominent and nearly circular. The