OSSEOUS SYSTEM OF MAMMALIA. 



305 



190 



The steps by which the bony capsule of the otic organ is 

 finally differentiated and individualised in Mammals are instruc- 

 tive examples of that character of advance in organisation. The 

 ex- and par-occipitals which contribute a partial bony support to 

 the back part of the gristly cap- 

 sule in Fishes and Reptiles, and 

 coalesce with that fully ossified 

 capsule in Birds, remain distinct 

 from the petrosal in all Mammals. 

 The alisphenoid, which contri- 

 butes a partial bony support to 

 the fore part of the gristly otic 

 capsule in H^ematocrya, and co- 

 alesces with the same part of the 

 bony capsule in Birds, has like- 

 wise permanently liberated itself 

 therefrom in Mammals. The 

 mastoid, which contributes a bony 

 support to the outer part of the 

 otic capsule in cold-blooded Ver- 

 tebrates, and is extensively con- 

 fluent with the same part of the 

 ossified capsule in Birds, retains 

 such confluence in some Mam- 

 mals, but instructively manifests 

 its primitive independency in 

 others. In the Cetacea, where 

 the mastoid and paroccipital are 

 distinct from the petrosal, this 

 capsule coalesces with the tym- 

 panic, which, having lost its man- 

 dibular function, is fixed and 

 contracts anchylosis with the pe- 

 trosal. The Babyroussa exempli- 

 fies the essential individuality of 

 the acoustic capsule, the petrosal 

 not only being ossified from its 

 own centre, but remaining dis- 

 tinct from every bone of the oto- 

 crane.^ 



§ 177. General Characters of the Limhs. — The diverging append- 

 age of the occipital vertebra is never absent in Mammals, and 



Bones of fore-limb, Horse. 



XLiv. p. 556, nos. 3388, 3345. 



VOL. II. 



