EASTMAN : sharks' TEETH AND CETACEAN BONES. 95 



MYSTACOCETI. 



Balaenidae. 



A. Balaenine Section. — The tympanic is deep and more or less 

 rhombic, its iuBatiou comparatively sliglit, the involucrum (t. e., the 

 reflected superior portion of the inner wall of the bone) not fig-shaped, 

 and frequently with no well marked depression at the anterior extremity 

 of the superior border of the inner surface for the Eustachian canal. 



B. Balaenopteriue Section. — The tympanic is long, much inflated, 

 rounded, with the involucrum much thickened and more distinctly pyri- 

 form, and the notch for the Eustachian canal always well marked. The 

 tympanic varies iu diff"erent individuals of the same species much less 

 than in the Balaenine section. 



ODONTOCETI 



Physeteridae 

 The anterior facette of the periotic for articulation with the tympanic 

 is quite smooth ; the posterior tympanic surface of the former is broad, 

 and carries a median longitudinal ridge. Tympano-periotic rigidly 

 united with the cranium by a bony process. 



Hyperoodon 



The posterior portion of the periotic is shortened, the median ridge on 

 the tympanic aspect of the same very strongly developed, the accessory 

 ossicle {processus tubarius) large and rounded, and the anterior tympanic 

 facette slightly concave. 



Kogia 



Tympanic and periotic firmly united with each other by their anterior 

 processes only, the posterior processes being widely separated and em- 

 bracing between them a portion of the mastoid bone. Tympanic bulla 

 scarcely inflated, aperture for Eustachian canal vertically constricted, 

 sigmoidal process knob-like and prominent. The apertures seen on the 

 inner aspect of the periotic (that which is turned away from the tym- 



of which the author is speaking, and the one most commonly occurring in the fossil 

 state. Amongst Odontocete whales and Delphinoids, on the other hand, the more 

 important distinctive characters are furnished by the periotic. The analysis here 

 given for the family divisions is taken chiefly from Dr. R. Lydekker's Catalogue of 

 the Fossil Mammalia iu the British Museum (1887). 



