1843.] 285 



Petro-tympanic bone of Balasna affinis.* 



One of the most complete of the fossil tympanic bones, which 

 measures five inches in length, resembles the Bal. antarctica in the 

 slight elevation of the posterior part of the involuted convexity and its 

 gradual diminution to the Eustachian end of the cavity: it resembles 

 both BalcencE in its traceable continuation to that end, and in the 

 gradual continuation of the concave outer wall from the involuted 

 . convexity ; this convexity is indented also, as in both recent Ba- 

 lenfe, by vertical fissures nai'roAver than the marked indentation 

 which distinguishes the Bal. mysticetus : these fissures are almost 

 worn out by friction in some of the specimens. The more perfect 

 one under consideration is not, however, identical with the Bal. 

 antarctica. 



The upper surface of the bone maintains a more equable breadth 

 from the posterior to the anterior end, the outer angle of which, 

 being well mai'ked in the fossil, is rounded off in the recent si^eci- 

 men ; the under and outer surfaces of the tympanic bone meet at 

 an acute angle. The above characters are sufiiciently marked in 

 the specimens of the fossil tympanic bones to justify their being 

 regarded as belonging to a species distinct from the known exist- 

 ing Balcence, but nearest allied to the Bal. antarctica, and which 

 I propose to call Balcena affinis. 



A second species is more uneqviivocally indicated by the distinct 

 definition of the involuted convexity ; and the extent of the slightly 

 concave surface extending from it to the commencement of the 

 overarching wall ; the anterior extremity of the involuted convexity 

 is equally well defined, and a wide concavity divides it from the 

 anterior extremity of the Eustachian cavity. The thickest part of 

 the involuted convexity is not very prominent. The under and 

 outer surfaces of the bone meet at a right angle. 



The species indicated by the tympanic bones of this form may be 

 termed BalcEna definita. 



A third form of tympanic bone differs from the first in the 

 shorter and more convex foi"m of the involuted part, the anterior 



* This engraving is one of a series (ilhistrating each of the four species) 

 prepared for Prof. Owen's work on the " British Fossil Mammalia and Birds " 

 now in course of publication. The cut was very kindly lent by the publisher 

 (Mr. Van Voorst) to illustrate this paper in the " Proceedings." — Ed. 



