INTRODUCTION. 19 



continued into the keel ; the posterior end shows a differentiated 

 ridge. Entrance to the cavity has a thick, rounded, striated border, 

 with somewhat depressed notch at Eustachian end. The keel is not 

 prominent, and blends with the roughened adjoining inner and outer 

 surfaces. 



The Tympanic in the Odontoceti is readily distinguishable from 

 this bone in the Baleen Whales ; on the inner surface the lower 

 border of the entrance to the tympanic cavity is not horizontal or 

 nearly so, but slopes downwards and forwards, so that the Eustachian 

 end is immediately above the anterior end of the inferior surface. In 

 Physeiev, a toothed whale of gigantic proportions, the bone is smaller 

 even than in B. rostratit, which has scarcely half the bulk of the 

 Sperm Whale. Even in an adult male Cachalot its length was only 

 2|^ inches. Its outer surface in the posterior part is raised into a 

 strong, curved ridge, continuous with the mallear or lip-like process 

 on the upper border, and prolonged downwards to the inferior surface. 

 The inferior surface is grooved and bilobed behind ; the outer is larger 

 than the inner lobe, roughened in front, above the anterior end 

 of which the Eustachian mouth of the cavity opens by an arch- 

 shaped orifice. In Kogia the tympanic is not raised on the outer 

 surface into a prominent ridge as in Physeter. The bone is bilobed 

 posteriorly, the outer being the larger ; the inferior surface is smooth ; 

 the inner surface is bounded above by the rounded border of the 

 opening into the cavity, which is divided by a shallow depression 

 into an anterior and a posterior part : the Eustachian end of the 

 cavity opens by an arched border. 



In the Zipliiinse the tympanic varies in its characters. In 

 Hyperoodon it is almost as large as in the sperm whale ; the upper 

 border of the outer surface gives rise to a curved lip-like process, but 

 its extension down the surface is feeble. A well-marked outer lobe 

 is behind, but the inner is a rudimentary tubercle ; a narrow rough 

 ridge, forming a low keel, projects from the inferior surface, close to 

 the Eustachian opening at its anterior end. In Ziphius the lip-like 

 process is limited to the upper border of the outer surface ; the 

 inferior surface has a low keel which ends behind in. the projection 

 or outer lobe, though the inner lobe is repi'esented by only a minute 

 tubercle ; the Eustachian opening is close to the anterior end of the 

 inferior surface. In Mesoplodon, again, the tympanic is definitely 

 two-lobed behind — the outer, the larger, smooth and rounded ; the 

 inner well marked, separated from the outer by a shallow cleft ; the 

 inferior surface, relatively l)road, is not keeled but grooved, and the 

 Eustachian opening is close to its anterior end. 



In Platanista the tympanic is 2 inches long, bilobed posteriorly, 

 the outer being the larger lobe ; inferior surface is grooved and is 

 prolonged into a pointed process, immediately above which is the 

 Eustachian opening of the tympanic cavity. The lip-like process 

 projects feebly from about the middle of the upper border. 



In the Deiphinid.Tp the tympanic is bilobed posteriorly, and the 

 outer is larger than the inner lobe. The lobes are best seen on the 



