THE MAMMALIAN OSSICULA AUDITUS. 189 



The Marsupialia are distinguislied for the uniformly low type 

 of their ossicula, although no single feature indicating inferior 

 grade in them is not to be found, in isolated cases, in higher 

 mammals. These bones are of the most iU-developed consistence 

 in the Peramelidse, and of the highest form in the Didelphyidse, 

 where the incus has a well-developed processus brevis, and the 

 stapes, alone among Marsupials, is perfectly bicrurate. In the 

 Kangaroos the ossicula are central in character, the malleus bears 

 a large foliaceous processus gracilis, as in the Wombat ; but the 

 stapes is always partially bicrurate. In the Phalangistidse the 

 stapes is generally columelliform, or only slightly bicrurate, the 

 incus is of as high a type as in the American Opossum, the mal- 

 leus is of the form seen in a new-born Macropus ; this ossicle 

 has distinctive feabures in Pliascolarctos. 



In the Wombats the malleus is not so much of high type as an 

 extreme form of the development of that bone in. Macropus. The 

 incus has a stapedial crus somewhat like that of Perameles ; the 

 stapes is always columelliform. In tne Dasyures, including 

 Phascogale and Myrmecolius, the incus is of low type, and the 

 stapes columelliform ; but all their ossicles are of more solid con- 

 sistence than in the Bandicoots. 



The distinguishing features in the ossicula of the Monotremata 

 are a peculiar form of articulation between the malleus and incus 

 by means of a scale-like development from the head of the former, 

 and the presence of an absolutely columelliform stapes. Yet the 

 three ossicula are not much modified from their representatives in 

 the lower Marsupials. The incus is ankylosed to the malleus in 

 Echidna and generally in the adult OrnithorliyncTius, but not in 

 the same manner as in certain Eodents. These bones are of more 

 solid consistence in Echidna than in the other Monotreme. In 

 all cases among mammals where the stapes is columelliform, it 

 clearly represents the entire bicrurate stapes of most animals in 

 that class, but, according to the most recent views of embryolo- 

 gists, only a part of the columella of birds and reptiles, the greater 

 part of that long ossicle in the latter classes probably represent- 

 ing the long process, or at least some other portion, of the incus. 



