Fig. 12
From: From Land to Water: the Origin of Whales, Dolphins, and Porpoises
Ectotympanic bones of Pakicetus and the modern dolphin Lagenorhynchus. This bone surrounds the middle ear cavity like a bowl. In all cetaceans, the medial wall of the ectotympanic is very thick, as indicated by the white line, and is called the involucrum