SKELETON OF MARSUPIALIA. 361 



with the tibia, fibula, calcaneum, c, and na\dculare, n. The upper 

 articular surface for the tibia is as usual concavo-convex, the in- 

 ternal surface for the inner malleolus flattened and at rio-ht anofles 

 with the preceding, but the outer articular surface presents a tri- 

 angular flattened form, and instead of being bent down parallel 

 with the inner articular surface, slopes away at a very open angle 

 from the upper surface, and receives the articular surface of the 

 fibula, 67, so as to sustain its vertical pressure. A small propor- 

 tion of the outer part of the inferior surface of the astragalus 

 rests upon the calcaneum : a greater part of the superincumbent 

 pressure is transmitted by a transversely extended convex anterior 

 surface to the naviculare, n, and cuneiform bones, i, e. This 

 form of the astragalus is also characteristic of the Koala, Petau- 

 rists, Dasyures, and the Pedimanous Marsupials generally. In 

 the Kangaroos, Potoroos, and Perameles which have the pedes 

 saltatorii, the fibular articular surface of the astragalus is bent 

 down as usual at nearly right angles with the upper tibial surface. 



The calcaneum in the Wombat presents a ridge on the outer 

 surface which serves to sustain the pressure of the external mal- 

 leolus, which is not articulated to the side of the astragalus. The 

 internal surface which joins the astragalus is continuous with 

 the anterior slightly concave surface Avhich articulates with the 

 cuboides. The posterior part of the bone is compressed, it pro- 

 jects backward for nearly an inch, and is slightly bent down- 

 ward and inward. This part is relatively shorter in the Koala, 

 Phalangers, Opossums, and Petaurists, but it is as strongly de- 

 veloped in the Dasyuri as in the Wombat. The anterior part of 

 the calcaneum of the Phalangers is shown at c, fig. 228. 



In the Dasyurus macrurus a small sesamoid bone is wedged 

 in between the astragalus, tibia, and fibula at the back part of 

 the ankle-joint. In the Petaurus tac/uanoides there is a supple- 

 mental tarsal bone wedged in between the naviculare and cuboides 

 on the plantar surface. 



The homotypy of the carpal and tarsal bones is very clearly 

 illustrated in the Phalanger. The lunare and scaphoid of the 

 hand correspond with the astragalus and naviculare of the foot, 

 transferring the pressure of the focile majus upon the three 

 innermost bones of the second series. The long, backward- 

 projecting pisiform bone of the wrist closely resembles the pos- 

 terior process of the os calcis ; the articular portion or body of 

 the OS calcis corresponds with the cuneiforme of the carpus ; the 

 large carpal unciform represents the tarsal cuboides, and performs 

 the same function, supporting the two outer digits ; the three 

 cuneiform bones of the foot are obvious homotypes of the trape- 



