2.o6 



THE QUARTERLY REVIEW OF BIOLOGY 



connected with the organ of hearing. It 

 is a finger-shaped structure, measuring 

 about 3$ inches in length, and is soft and 

 flexible in a fresh state. Covering the 

 bulla is a layer of elastic and fibrous 

 tissue some three inches in thickness, 

 which exudes fat, and this is covered by a 

 4 inch outer layer of spongy tissue with 

 air spaces. To the latter is applied the 

 muscular wall of the pharynx. Thus 

 nearly a foot of tissue separates the 

 tympanic bulla from the cavity of the 

 pharynx. The periotic bones of living 

 and fossil whalebone whales are all 

 alike in that they have a long posterior 

 process, which is wedged in between the 

 exoccipital and squamosal bones. 



Although the smaller porpoises are 

 not accustomed to dive to such great 

 depths as the whalebone whales, many 

 of them have a partially calcified tympanic 

 membrane. Yet in all these toothed 

 whales the external orifice is so minute 

 that it is practically closed. The rigid 

 malleus articulates in the usual mam- 

 malian way with the incus, the long 

 crus of the latter in turn with the head of 

 the stapes, and the footplate of the stapes 

 is sunk into the fenestra ovalis and arrested 

 by the lining membrane of the vestibule. 

 Notwithstanding these osteological obsta- 

 cles, we do have evidence indicating that 

 porpoises are irritated if not repelled by 

 certain kinds of water-borne sounds. 

 The conductive portions of the organ of 

 hearing may be defective in certain 

 respects, but the essential parts are 

 retained. The cochlea is of the spiral 

 type and varies from one and three- 

 quarters to two full turns. In contrast to 

 the whalebone whales, the porpoises, 

 dolphins, sperm whales, beaked whales, 



and their relatives all have the ear bones 

 attached to the skull by ligaments. 



Zeuglodonts have been assumed to 

 represent a part of the shallow water 

 fauna of the Eocene period, and yet they 

 have an organ for hearing of the type 

 commonly associated with whales accus- 

 tomed to diving to considerable depths. 

 We have indisputable evidence that the 

 tympanic bulla was in existence in this 

 group of pelagic mammals at the begin- 

 ning of the Middle Eocene age, for Proto- 

 cetus has a bulla as fully developed as ; 

 any living cetacean. Both the whalebone 

 and the toothed whale types of periotic 

 bones have been found associated with 

 skulls of zeuglodonts. The cochlea is 

 similar to those of living whales, and the 

 anterior process of the malleus is fused 

 with the bulla. It is thus apparent that 

 some of these Eocene zeuglodonts had a 

 rigid malleus and that the same limita- 

 tions are placed upon the functioning of 

 the organ of nearing as in most living 

 cetaceans. 



No matter how defective the cetacean 

 organ of hearing may appear in com- 

 parison with those of land dwelling 

 mammals, we have evidence that these 

 modifications were present in some if not 

 all of the earliest known zeuglodonts. 

 If the conch-like bulla with its rigid 

 malleus and pedicle method of attachment 

 to the periotic arose in response to the 

 requirements of an aquatic mode of life, 

 these structures probably meet the exigen- 

 cies of their environment, and are adapted 

 for the perception of water-borne vibra- 

 tions, for it is unlikely that so many 

 diversified types of cetaceans would have 

 survived until the present time if this 

 organ were unsuited for their purposes. 



