OF THE MAMMALIAN OSSICULA AUDITUS. 463 



forward from it. Hence the malleus of Inia allies it, on the whole, more to Delphvnus 

 than Platanista. The articular surface for the stapes on the stout processus longus 

 is as wide as in Delphmus; the processvis hrevis is rather long, hut slender. The 

 stapes is of the type prevailing among all the Dolphins ; it has an intercrural aperture. 

 In a full-sized adult Inia these ossicula are less than half as large as their -representatives 

 in a Platanista of the same age and proportions. 



In taking a general concluding view of the auditory ossicles of the Cetacea, it may be 

 remarked : — 



I. The ossicula of these animals differ from those of other Mammalia rather in their 

 solidity than in any great size in proportion to the whole skeleton, and more in extreme 

 modifications of detail than in solidity of composition ; for in size they are exceeded by 

 the Seals and Sirenia, in density by the latter, but in specialization of form by no other 

 mammals. 



II. The general diagnostic features are : — in the malleus, constant, firm bony ankylosis 

 to the tympanic bone through the medium of the processus gracilis, with ill-development 

 or complete suppression of the manubrium; in the incus, great development of the 

 stapedial crus ; in the stapes, thickness of the crura contracting or obliterating the aperture. 

 This latter bone always fits firmly into the fenestra ovalis, to which, however, it is in 

 no species constantly ankylosed. 



III. The genus Balcena possesses undoubtedly the most generalized ossicula among 

 all the Cetacea, particularly in the case of the incus. The malleus has the least-modified 

 form of manubrium, the whole bone distantly resembling that of certain large Seals 

 (Macrorhinus, Stenorhynchus), as does also the stapes. 



IV. The malleus of Neobalcena is the next least specialized. That of Megaptera is 

 much more modified in regard to the aborted condition of the manubrium. 



V. Both in the malleus and incus Balcenoptera is far more modified than any of the 

 above genera*. The incus is quite Delphinoid, unlike either the Balcena or Physeter 

 type. The malleus is even more modified than in the Dolphins ; its manubrium is of more 

 tubercular form, and the whole bone is more flattened. 



VI. In the Physeteridse the malleus is extremely modified. The tubercle projecting 

 from the head in other Cetacea is here obsolete ; the manubrium and processus muscu- 

 laris are reduced to two small spines. That representing the manubrium is almost sup- 

 pressed in Physeter, Myperoodon, and Mesoplodon, but is well developed in Berardius. 

 The head is largest in Berardius ; in Physeter the articular surface is very extensive. 

 The incus is intermediate in type between the same in Balcena and Delphinus. In the 

 the Ziphioid Whales the short crus is long and of peculiar shape. The stapes differs a 

 little in character from that of the true Dolphins. 



VII. In the Dolphins the malleus is never so modified as in the Ziphioid Whales : the 

 tubercle projecting from its head is pointed, not rounded as in Balcenoptera ; and the 

 seat of insertion of the tensor tympani muscle is indicated by a roughened, depressed 

 surface. The manubrium may be quite absent {Delphinapterus) or reduced to a small 



* See Appendix to this section. 



3q2 



