THE MAMMALIAISr OSSICULA ilTDITUS. 187 



to it ; an orbicular apophysis, observed in Sorex, is also con- 

 stant, but not pedunculated. The manubrium is quadrilateral, 

 the inner as well as the outer aspect being sharply bordered off 

 from each of the sides. The incus is very similar to that of the 

 Shrews and Marsupials : the stapes is of a rather higher type than 

 the other ossicula; its aperture is generally wide and occupied in 

 the recent skull by a small artery unsupported by any bony 

 canal. 



In the Cetacea the malleus is constantly united to the tympanic 

 bone by firm bony ankylosis through the medium of the processus 

 gracilis. The manubrium is ill developed or completely sup- 

 pressed. The stapedial cms of the incus is greatly developed. 

 The crura of the stapes are thick and encroach upon or obliterate 

 the aperture. This ossicle always fits firmly into the fenestra 

 ovalis, to which it is in no species coustantly ankylosed. 



These bones are most generalized in type in the genus Balcena, 

 particularly as regards the incus ; the malleus, too, has the least- 

 modified form of manubrium. Next in order come JSfeohalcena 

 and Megaptera ; but Balcenoptera possesses a malleus and incus 

 as modified as in the Dolphins. In the Physeteridse the malleus 

 is extremely modified, the manubrium and processus muscularis 

 are not borne upon a tubercle projecting from the front of the 

 head as in other Cetacea. but are reduced to two small spines. 

 That representing the manubrium is almost obsolete in Pkyseter, 

 Hyperoodon, and Mesoploclon, but fairly developed in Berardius. 

 The incus has characters intermediate between the same in Ba- 

 lcena and Delphinus ; the stapes assumes a tolerably distinctive 

 form. 



In all the Dolphins the incus has a very large stapedial crus and 

 a stapes with stouter crura than in the Whale-bone and Sperm- 

 Whales. The malleus resembles considerably that of Balceno- 

 ptera ; the manubrium may be represented by a spine bent down- 

 wards (Orea, Pseudorca) or depressed against the side of the o%- 

 siele (^GlobiocepJialus, LagenorhyncJius, PJioccena) ; or that spine 

 may be almost or quite obsolete {Delphinus, Delphinapter us) -. part 

 of the process from the membrana tympani is inserted along a 

 narrow groove in the tubercle. Monodon has a spine-like manu- 

 brium, but differs from the other Dolphins in the characters 

 of its articular surface and the length of the head and tubercle. 

 The head of the malleus is best developed in Orca. 



17* 



