OF THE MAMMALIAN OSSICULA AUDITUS. 457 



The incus of Physeter has a much smaller body than that of Balcena; but this 

 portion of the ossicle is, on the other hand, not so reduced in size as in Balcenoptera and 

 the Dolphins. The articular surface is very wide ; the upper facet is the larger and 

 planer, the lower is very concave, to fit the corresponding parts on the malleus. The 

 ridge dividing them is most distinct anteriorly. The processus brevis is slender, but 

 rather long ; the processus longus is very stout, but not so big as in the Delphinoidea 

 and in Balcenoptera ; it bears at its extremity a small, sessile, elliptical Sylvian 

 apophysis. 



The stapes has extremely stout crura, not so divergent as in the Dolphins ; the aperture 

 is sometimes obliterated. The footplate has a very thick border, and is a little con- 

 cave towards the vestibule. I have never found this ossicle ankylosed to the petrous 

 bone. 



In the Ziphioid Whales the ossicula much resemble those of Physeter. 



In Hyperoodon rostratus (PL LXII. fig. 35) the malleus has a larger head than in the 

 Cachalot ; that portion of the ossicle is also more convex, and rises a little above the 

 upper border of the articular surface. 



There is hardly any trace of a depression on the outer aspect of the head ; it seems 

 rather as if the bone above had grown into it, and reduced it to a horizontal slit. The 

 process holding the malleus to the petrous bone is extremely short, but its homologies 

 can be easily made out. The articular surface is, proportionately to the rest of the 

 ossicle, considerably smaller than in Physeter, nor is the upper so much larger than the 

 lower facet. 



The processus muscularis (PL LXII. fig. 35, pm) is even less developed than in 

 Physeter ; the manubrium (mti) is a still more insignificant tubercle. 



The incus also in general characters resembles the same in the Cachalot, but possesses 

 a character which it shares with the two other Ziphioid Whales whose ossicula the author 

 has examined, and in which it differs from Physeter. This feature is the long, flattened, 

 and very divergent processus brevis, which is narrowest in the middle. The articular 

 surface has its facets divided by a very sharp ridge, equally defined in its whole course ; 

 the upper facet is plane, the lower concave, as in Physeter, but consistently with the 

 characters of the articular area in the malleus of Hyperoodon the upper facet is, in its 

 incus, not so much larger than the lower, as is the case in the Cachalot. The body is 

 developed to the same extent as in Physeter ; the processus longus is as stout but 

 shorter than its representative in that Whale. 



The stapes in Hyperoodon (PL LXII. fig,. 35) has very stout crura, with hardly a 

 vestige of an aperture. The base is narrower than in Physeter, and not nearly so thick at 

 its margin ; towards the vestibule it is quite concave. 



The stapes, but not the other ossicula, of Ziphius cavirostris is figured in a plate 

 illustrating Prof. Turner's paper on that Cetacean *. It resembles, apparently, that of 

 Hyperoodon. 



The malleus from the fine adult specimen of Berardius arnuxii in the College Museum 

 (PL LXIII. figs. 1 & 18) bears, to a well-marked degree, all the chief peculiarities seen 



* Trans. Royal Soc. Edinb. 1872. 



