480 MR. A. H. G. DORAN ON THE MORPHOLOGY 



have a malleus of the type of Macropus, Priodon, or Rhinoceros ; let the head remain 

 ■undeveloped, and the lamina take an extra growth, then the laminated type will be seen 

 in the full-grown animal ; hut if the head and neck grow, they must necessarily encroach 

 on the lamina, and the adult will have a malleus like that of Man. 



The foetal malleus of Dasypus or Tatusia, very Macropoid in character, undergoes the 

 latter change, whilst in Tolypeutes it grows into a laminated hammer, and in Priodon 

 remains unchanged in the adult. Almost similarly the mallei of the adult Rhinoceros 

 and Hquus differ, whilst that of the foetal Horse is remarkably like the adult Rhinoceros's. 

 Again, the mallei of foetal Ruminants ultimately overshoot the latter animal in type, 

 but do not ultimately reach the high form of Equus. 



The incus of Macropus (PI. LXIV. fig. 27) lies in a very similar position to that of 

 many Edentates, the processus brevis being almost superior ; and the stapedial crus is 

 directed rather backwards as well as inwards. The body is very small in proportion to 

 the crura, and deeply excavated by the articular surface. The processus brevis is stout 

 and blunt-pointed, rather long, and little divergent from tlie body. The processus longus 

 is long, and rather slender, it is not much curved, and supports a very distinct almost 

 circular Sylvian apophysis on a narrow pedicle ; altogether it more resembles the singular 

 incus of Perameles than that of any Monodelphian mammal. 



The stapes (figs. 27 & 42) has a small head and two slender crura united for halfway 

 towards the base, where they suddenly diverge, leaving a distinct triangular aperture ; 

 the outline of the crura is slightly concave. There is frequently a spur-like process for 

 the insertion of the stapedius-tendon, rather on the inner crus than on the head. The 

 base, which projects considerably beyond the insertion of the crura, is much more nearly 

 circular than in most placental mammals ; the upper border particularly is very much 

 curved. 



All these characters are seen almost literally imitated in Tolypeutes. I deem it hardly 

 necessary to waste time in showing at length how this type is intermediate between the 

 bicrurate stapedes, with large apertures, as in Homo, and the columellif orm type of the 

 Ornithodelphia ; still less is there any occasion to demonstrate which is the lowest, and 

 which the intermediate form, that being seen at a glance if a Mau's, a Kangaroo's, and 

 an Echidna's stapedes be put in a row. The Macropoid type of stapes is approached, it 

 must be remembered, in animals as high as the Marmosets. 



All the ossicula of other species of Macropus, as well as those of Hypsiprymnus and 

 Lagorchestes, resemble the same in M. major. The crura of the stapes divide in a 

 similar manner, and the base is broad and flat ; nor has it the least tendency to become 

 bullate in the smaller Kangaroos, such as Hypsiprymnus murinus (PI. LXIV. figs. 27, 

 28, 29, & 30). 



The Didelphyid^e will now be described, as they resemble the Kangaroos in that the 

 stapes is not completely columelliform. 



The malleus of the Opossum of the United States {Didelphys virginiana, PI. LXIV. fig. 15) 

 is a little smaller than the same ossicle in a small Hypsiprymnus ; in its characters it closely 

 resembles that of Macropus. The head is ill developed, and the articular surface deeply 

 cut, with facets of the same character as in the family just described. The neck is of the 



