skull in placental mammals. The ectotympanic bone which supports the eardrum is similar to that of Pakicetus, about as long as wide, whereas later archaeocetes have more elongate ectotympanics. The ectotympanics of all archaeocetes, nonetheless, are much different than those of terrestrial mammals. The ectotympanics of all cetaceans, including Ambulocetus, possess an involucrum (thickened lump of bone) at the medial lip. Unlike Pakicetus, but like later archaeocetes, the tympanic made close contact with the jaw. Like later archaeocetes, Ambulocetus seems to have possessed an air sinus in the pterygoids. It may have also had paranasal sinuses. The parietal bones on the braincase sides are more perpendicular than in Remingtonocetus, which makes the cheeks appear less flared. Like Remingtonocetus, Ambulocetus appears to have had a small brain. The snout was quite broad, but the end of the holotype's snout is missing, so it is unclear how long it would have been. The snouts of Basilosaurus and Rodhocetus are short and make up about half the skull's length. Remingtonocetid snouts are quite narrow, which was clearly not the case for Ambulocetus. The mandibular symphysis of most mammals is restricted to the midline of the jaw, but extends much farther in archaeocetes; in Ambulocetus, it reaches the back end of the first premolar. Snout robustness and symphysis length suggest reinforcement of the jaw to withstand a strong bite force. Similarly, the strongest biting muscle in Ambulocetus seems to have been the temporalis muscle involved in biting down. Like other cetaceans, there are embrasure pits (a depression between the teeth), preserving the tooth positions for the fourth premolar, the first molar, and the third molar. Unlike later archaeocetes, the molars' roots do not extend to the cheek bones, and the third molar is not as nosewards as in remingtonocetids. The coronoid process of the mandible (where the lower jaw connects with the skull) in Ambulocetus is steep. In contrast, it is low and slopes gently down in basilosaurids and later cetaceans. The mandibular foramen opens below the coronoid process, and is around midway between terrestrial mammals and toothed whales in size. Like other cetaceans, the body of the hyoid bone (the basihyoid bone) is about as long as wide. Unlike other archaeocetes, the eyes are quite large and are placed near the top of the head facing upwards. Unlike modern toothed whales which only have one kind of tooth (homodont), archaeocetes are heterodont. Judging