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1451. A cast of a mutilated right tympanic bone of the Bal(E?ia definita, Owen. 



This species is more unequivocally characterized than the preceding, by 

 the distinct definition of the involuted convexity, and by the extent of the 

 slightly concave surface extending from the convexity to the commence 

 ment 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 outlet. The involuted convexity 

 is but slightly elevated even at its broadest part. The under and outer 

 surfaces of the bone meet at a right angle. 

 From the red crag at Felixstow, Suffolk. 



Presented by the Rev. Prof. Henslow, F.R.S. 



1452. A mutilated right tympanic bone of the Bala7ia definita : it equally well ma- 



nifests the characters of the well-defined convexity, and the wide interspace 

 between that part and the origin of the overarching plate, most of which is 

 broken away : in this example also the under and outer surfaces of the 

 bone meet at right angles to form a ridge along the exterior of the bone. 

 From the red crag at Felixstow, Suffolk. 



Presented by the Rev. Prof. Henslow, F.R.S. 



1453. A mutilated right tympanic bone of the Balcena gibbosa, Owen. This 

 differs from the tympanum of the Balcena affinis in the shorter and more 

 prominent involuted convexity, the anterior end of which is divided from 

 the anterior end of the cavity by a concave border of nearly equal longitu- 

 dinal extent; the internal border of the involuted convexity is also better 

 defined than in the Balcena affinis, but is situated as closely as in that 

 species to the overarching wall, which is divided from it only by a deep 

 and narrow rugged fissure, and not by a broad and gently concave tract 

 as in the Balcena definita. Both the outer and under surfaces of this 

 specimen are more rounded than in the two preceding species ; but, being 

 more mutilated and waterworn, the characters derivable from 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 

 distinction of the present fossil. 



From the red crag at Felixstow, Suffolk. 



Presented by the Rev. Prof. Henslow, F.R.S. 

 2 P 



