You should look at the work of Gerald Fleischer, J. of Paleontology vol 50 #1 p133-52, Jan 1976: "Hearing in extinct cetaceans as determined by cochlear structure" and N. Jb. Geol. Palaont. Abh 151 2 pp 166-91. Konstruktionsmorphologie, Nr. 56 "On bony microstructures in the dolphin cochlea, related to hearing" G. Fleischer, Tubingen Germany. May 1976. Jont Allen