176 PROFESSOR STRUTHERS. 



tympanic, has projecting into it, for about J inch, the nipple- 

 like process. The anterior boundary is formed at its upper f 

 by the concave outer edge of the lamella of the tympanic. 

 At the upper J of this boundary the bony meatus opens into 

 a fissure, f inch long, J inch wide, between the periotic 

 and tympanic, closed in front by the hinder edge of the an- 

 terior junction, near to which the fissure becomes suddenly 

 narrowed. 



The lower bony wall of the passage, from its outer end to the 

 posterior division of the opening of the tympanic bone, is short, 

 J inch behind, at the nipple-shaped process ; in front of this it 

 is a mere edge. The smooth concave surface on the front of the 

 posterior junction is 1 inch in height as well as in breadth, and 

 has that length as a wall of the meatus in to the very back of 

 the opening of the tympanic bone, but about half that smooth 

 surface is external to the plane of the upper boundary. The 

 distances from the outer end of the meatus to the promontory, 

 just below the fenestra ovalis, are at anterior and upper walls 

 of meatus 1J inch, at posterior wall If inch, taken from the 

 outer edge of the smooth surface. The fenestra ovalis is oppo- 

 site the upper posterior corner of the meatus externus. It 

 lies at the bottom of a conical recess, about J inch deep, into 

 which the stapes is sunk. The stapes moves freely, but it is so 

 held in the fenestra, at the apex of the fossa, that it cannot be 

 removed. 



[In B. musculus the smooth anterior surface of the posterior junc- 

 tion, bounding the meatus behind, presents a very different character. 

 It forms a deep rounded fossa, about f inch in diameter and ^ inch 

 deep, receiving the end of the thumb, and opens in front into the 

 narrow posterior end of the opening of the tympanic bone. In 

 Megaptera the concavity is shallow, about ^ inch deep.] 



24. THE MANDIBLE. The following table (Table II.) brings 

 out various differences on the mandible of the two species. In 

 comparing the form, the parts to be observed are the coronoid 

 process, the condyle, the elongated neck between these two 

 processes, and the body in front of the coronoid process. 



