OF THE MAMMALIAN OSSICULA AUDITUS. 481 



same slender build ; it is very straight until close to the manubrium, where it curves 

 suddenly. The lamina is decidedly wider than in any Kangaroo, and prolonged upon 

 the processus gracilis, forming a curved process applied, as in other marsupials, to the 

 tympanic ring very closely, though readily detached from the latter even in the oldest 

 male specimens, without the aid either of a lens or of any great skill in dissection. Tbis 

 process is of more regular form in Didelpliys tban in Macropus, being of nearly equal 

 breadth throughout except at the extremity, which is slightly dilated. No foramen can 

 be detected in it, as in JErinaceus, which possesses a somewhat similar but still more 

 uniformly broad processus gracilis. 



The manubrium is even more slender and straight than in Macropus ; but at the outer 

 angle of its base, the site of the processus brevis in Homo, it is peculiarly broad and flat- 

 tened, projecting towards the neck on the outer aspect. The extremity is almost styli- 

 form; still there is a distinctly bordered though extremely narrow outer surface*, which 

 may be seen by a magnifying glass to be slightly dilated towards the tip. This malleus 

 may be said to be a trifle higher in type tban that of the Kangaroo, exhibiting certain 

 affinities to some Insectivora. 



The incus of the Virginian Opossum has a deeper body than in the Kangaroo, with the 

 articular surface as deeply cut ; the processus brevis is thin, long, and rather divergent 

 from the body, lying almost vertically above the latter in the recent skeleton. The pro- 

 cessus longus is very long, and bears at its extremity a circular Sylvian apophysis on a 

 very long peduncle, which is broad, and flattened where it joins the extremity of the 

 apophysis f. A high development of this process is frequent among the Insectivora ; but 

 it is also seen in other families of the Marsupialia, where the peduncle is of similar form. 



The stapes has a small head, sometimes bearing a tubercle for the stapedius-tendon. 

 The slender crura diverge rather higher than in Macropus, so that the aperture is more 

 distinct and more triangular. The base, which is thin and flat, projects far beyond the 

 crura, is more oval, and hence not so deep vertically as in the Kangaroo (PL LXIV. 

 figs. 15 & 41). 



The stapes of Didelpliys is certainly of a higher type than that of Macropus or any 

 other marsupial, both in the greater divergence of the crura and in the characters of the 

 base. It much resembles that of some small American Monkeys and of certain Rodents 

 and Insectivora. Classifying the different types of stapes on the same principle upon 

 which the mammalian malleus was classified after describing that ossicle in Macropus, 

 it may be asserted that we find two extreme types : — on the one hand the stapes of 

 Homo, the Simiidse, and some Rodents (as Arctomys) and Insectivora %, where the crura 

 are freely divergent and curved with a large aperture; and on the other hand the 

 completely columelliform stapes of the Ornithodelphia, Phalangistidse, and Manidae. 



* See the description of the ossicula of the Carnivora, anted. 



t This apophysis is, of necessity, very easily broken off in preparing the ossicula for anatomical observation. 



X The stapes in these latter and clearly inferior animals has a much less-developed head than in Homo; and the 

 wideness of the aperture is necessary for the transmission of a bony canal. It may be said to constitute another 

 and a rather high type. 



