532 



Tympanic bone of Baleenu, gibbosa, nat. size. Felixstow. 



A third form of tympanic bone differs from the first in 

 the shorter and more convex form of the involuted part 

 (a), the anterior end of which is divided from the anterior 

 end of the cavity by a concave border one inch in extent ; 

 the internal border of the involuted convexity is also better 

 defined than in Bal. affinis ; but the overarching wall (5) 

 begins to rise close to it, divided from it only by a deep 

 and narrow rugged fissure, instead of by a broad and gently 

 concave tract, as in Bal. definita. 



Both the outer and under surfaces of these specimens 

 are more rounded than in the two preceding species ; but 

 being more mutilated and water-worn, the differences 

 of the external parts of the bone are of less value. The 

 characters above specified, which are furnished by the 

 involuted convexity, are decisive as to the specific distinc- 

 tion of the present fossil tympanic bone, and the third 

 species of extinct whale, so indicated, I have proposed to 

 call Baleena giblosa. 



