OF THE MAMMALIAN OSSICULA AUDITUS. 485 



(fig. 34) : it is rather broad at the base and flattened laterally, very slightly curved, 

 and dilated at the tip ; but above, at the site of the processus brevis, it is not only 

 broad, but presents a distinct concavity ; the raised margins (of which the outer appears 

 continued from the neck), very wide apart above, almost coalesce below, but keep slightly 

 apart towards the extremity, forming a very narrow outer surface to the manubrium *. 

 The incus of the Koala is smaller in proportion to the malleus than in the Phalangista ; 

 and the processus brevis is very short and ill-developed ; the stapedial cms bears a pedun- 

 culated Sylvian apophysis. 



The stapes is absolutely columelliforni, without any trace of an aperture ; it bears a 

 small bony process for the attachment of the stapedial tendon. 



Hence in all three ossicula Phascolarctos exhibits characters in which it differs from 

 Phalangista, and resembles rather more the majority of Marsupials. 



In the "Wombat (Phascolomys vombatus, PI. LXIV. figs. 32, 33) the most interesting 

 feature is the enormous development of the processus gracilis of the malleus. 



The head of the malleus is even less developed and more compressed laterally than in 

 the Dasyuridse ; it is much encroached upon by the articular surface, which is both 

 deeply cut and wide ; the lower facet, though more convex, is not much narrower than 

 the upper, from which it is divided by a well-marked groove. The neck is thin and 

 curved, as in the Kangaroos and Dasyures ; and the lamina is narrow. The manubrium, 

 as in most Marsupials, is slender, and forms little more than a right angle with the 

 neck. The outer aspect of the base is very broad, though not so modified as in the 

 Koala. The extremity of the handle is narrow but distinctly spatulate. The processus 

 gracilis, joined by the narrow lamina, forms a curious process three times as long as 

 the manubrium, which is closely applied to the tympanic bone, and can readily be 

 detached from it even in the adult. At first it is narrow and almost of as even breadth 

 as in the Phalangers ; but its distal two thirds form a broad sabre-like or leaf -like 

 process, sharp-edged, much flattened, and ending in a tapering point. The whole pro- 

 cessus gracilis is only an extremely developed form of the type seen in the Rhinoceros 

 and in many of the Insectivora and Marsupials. The foliaceous extremity is readily 

 broken off if the specimen be not carefully prepared. 



In the incus the body is fairly developed, especially above the processus brevis, though 

 the whole bone is not so large in proportion to the malleus as in Phalangista. The pro- 

 cessus brevis is rather short, but stout at the base ; it diverges but little from the body. 

 The stapedial cms is long and thin, and supports a large elliptical Sylvian apophysis on 

 a slender pedicle. The stapes is quite columelliform ; the head forms a narrow elliptical 

 capital to the fused crura ; just beneath it is a spur-like process, to which the tendon of 

 the stapedius may be found attached in the recent or even in the imperfectly macerated 

 skull. The base is not of the Sauropsidan character, being as narrow vertically as 

 in Homo. 



The ossicula of P. latifrons are very similar in character. They are all rather lar°-er in 

 the Wombats than in the largest Dasyures (such as D. ursinus) or Thylacinus. 



* The blunt outer aspect of the base of the manubrium in the mallei of large Dasyures and other Marsupials 

 exhibits an approach to the above-described peculiarity in the Koala. 



SECOND SERIES. — ZOOLOGY, VOL. I. 3 T 



