EASTMAN : sharks' TEETH AND CETACEAN BONES. 



93 



up the reflected inner wall of the bulla. The walls are thickest where 

 the surface is rough, and thinnest over the smooth outer lip. 



Posterior end (Fig. E). — Viewed from behind, the most marked 

 features are the bilobed form of the bulla, the large size and spongy- 

 texture of the posterior processes (4, 11) of the tympanic and periotic, 

 the continuity of the narrow tympano-periotic fissure (17), the large 

 canal for the egress of the facial nerve already referred to (20), and the 

 large opening in the posterior wall of the promontory known as the 

 fenestra cochleae (21). 



Anterior end (Fig. F). — Above is seen the obtuse forward extremity 

 of the periotic, below the produced and spout-like termination of the 

 Eustachian canal (19). The opening of the latter is not completely en- 



7--- 



Fig- E. — Left tympano-periotic of DeU 

 phinapterus leueas Pallas. Posterior end, 

 X I. 



Fig. F. — Left tympano-periotic of Del- 

 phinapterus leueas Pallas. Anterior end, 



X 1. 



closed, as in other Mammals, but drawn out into a long slit-like canal, 

 which is confluent above the superior margin of the inner lip with the 

 fissura tympano-periotica (17). A portion of the knob-like "accessory 

 ossicle " (6) of the tympanic is plainly visible below the overhanging 

 anterior process of the periotic (10). 



Having now examined different aspects of the periphery, our next pro- 

 cedure would be to separate the two elements of the tympano-periotic, 

 and observe those surfaces which are presented towards each other, and 

 hence remain concealed when the bones are in natural apposition. These 

 inner faces, however, are precisely the ones which have been oftenest 

 figured and described in the case of fossil forms, and as regards the recent 

 Delphinapterus, the general resemblance to Phocaena is such that the 



