392 



subsides into the rounded angle between the adjoining tibial facets, and is closely 

 applied thereto to near the distal end of the tibia. In this course the fibula is reduced 

 to a mere channelled plate, as shown in the transverse section (n a). The distal end 

 of the fibula again slightly expands, and contributes by its epiphysis an outer malleolar 

 buttress (ib. figs. 9, 11, b) to the mammalian ossifications in the tarsal segment of the 

 foot. 



The ossified tarsal segment is singularly modified in Macropodidac (PL LXXV.), in 

 relation to the concentration of the powers of the hind feet, as saltatory locomotive 

 instruments, upon the outermost pair of toes, and mainly upon one of these, answering 

 to the fourth in the pentadactyle foot. 



The narrowness of the hind foot as compared with the fore foot, which leads to the 

 displacement of the tarsal homotype of the carpal scaphoid to a more distal position in 

 its segment, is extreme in the Kangaroos — and being associated with atrophy of the 

 inner or tibial side of the metatarsus, renders still more obscure the true character of 

 the " naviculare " (PI. LXXV. fig. 1, n). The homotype of the carpal " lunare " almost 

 monopolizes the distal articulation of the leg-bones. In the tarsus it is the " astragalus" 

 (ib. figs. 3, 4, 5), and, in the Kangaroos, is subdepressed, triangular, with the base turned 

 forward (figs. 3 & 4) : viewed from the inner or tibial side (ib. fig. 5) it is arched, with 

 the convexity towards the leg. The upper surface (fig. 3) is chiefly formed by the 

 trochlear articulation ( l ) for the tibia, convex from behind forward, concave trans- 

 versely, passing into a convex outer or fibular border ( 2 ), but more definitely bounded 

 by a ridge (3) on the inner side, over which the articular surface passes, at almost a 

 right angle, to form the flattened one (ib. d, & fig. 5, d a) adapted to the inner mal- 

 leolus. Beyond this surface a depressed non-articular part of the bone (fig. 3, e) 

 extends a short way inward and forward, where it is terminated by the narrow anterior 

 oblong convexity (fig. 5,f) for the naviculare. The inner non-articular part of the 

 astragalus terminates in a tuberosity (ib. g). The posterior non-articular part, forming 

 the apex of the triangle (ib. h), applies itself to the hind part of the inner articular 

 expansion of the calcaneum, to which it is ligamentously attached. On the under 

 surface of the astragalus (fig. 4) is the antero-posteriorly concave surface (i), articu- 

 lating with the convex one on the outer half of the upper expanded articular part of 

 the calcaneum (fig. 6, i). Internal to this, at the middle of the under surface of the 

 astragalus, is the oblong flattened articulation (fig. 4, k) for the flattened surface on the 

 inner division of the upper articular part of the calcaneum (fig. 6, k). Anterior and 

 internal to this is a smaller facet (fig. 4, I), continuous with the navicular one (f), but 

 articulating with the inner side of the anterior calcaneal surface for the cuboid. No 

 part of the surface (f, I) is extended to the tibio-malleolar articulation as in Phasco- 

 lomys. The astragalar characters are well marked, and could not fail to be recognized 

 in a fossil bone of the Macropodal family. 



The calcaneum of a Kangaroo is equally well characterized (PL LXXV. figs. 1 & 2, 

 C, b, figs. 6 & 7). The fulcral portion (a b, a c) is long and triedral, the narrowest, 



