501 



it measures 13 lines in rotulo-plantar thickness, one third down ; and here, near the 

 plantar side, its thickness (or tibio-fibular breadth) is 6 lines. The opposite (rotular) 

 border is not sharp, as in Macropus major and Macr. rufus ; but though thin, in com- 

 parison with the plantar surface, the border is rounded off. The shaft loses rotulo- 

 plantar thickness and gains transverse breadth as it approaches the trochlear articular 

 surface (r); this is 1 inch transversely, 10 lines where thickest from before backward. 

 The surface is not simply convex, as in Macropus rufus (Plate CXVI. figs. 4 & 5, r), 

 but is made trochlear by a plantar median ridge, on each side of which the surface, 

 transversely, is feebly concave. The outer (fibular) side of the shaft has a feeble median 

 longitudinal channel along the middle of the proximal two thirds. The upper half of 

 the shaft shows in fractured portions of homologous fossils a small medullary cavity. 



Thus we learn that in the large extinct Kangaroos of the genus Palorchestes the 

 fourth and fifth digits were less unequal in strength, and the fifth took more share in 

 station and locomotion than in the largest existing kinds. The metacarpal segment 

 and the rest of the foot were proportionally broader; but the length of the fourth 

 metatarsal in Palorchestes indicates, nevertheless, that it was a powerful leaper. 



§ 21. Procoptodon (Metatarsals ir & r). — In deprecation of the foregoing details, 

 more wearisome perhaps to the reader than the inditer, I may plead the great propor- 

 tion of fragmentary evidences of the hind feet of large extinct Kangaroos as compared 

 with entire or nearly entire bones. The grateful aid which such rare specimens have 

 yielded has impressed me with the duty of defining and recording all characters which 

 may help future collectors, especially in Australia, in determining such fossil fragments 

 which are likely to accumulate in the public and private museums of that great colony. 



I have alluded to the primary step in the survey of the vast series of metatarsal 

 fossils which led to setting apart those indicative of a hind foot shorter in proportion to 

 its breadth, and yet retaining unmistakable macropodal characters. 



In the specimen, for example, of the naturally united fourth and fifth metacarpals 

 figured in Plate CIX. figs. 1-5, the fourth (ir) is thicker than, but is little more than 

 two thirds the length of, the homologous bone in Macropus rufus (Plate CXVI. fig. 4). 

 The fifth metatarsal (Plate CXXI. figs. 1, 2, 3, r) shows a greater degree of thickness, 

 in proportion to its length, than in Palorchestes Azael (Plate CXVI. figs. 1 & 2, r). 



The proximal end of the fourth metatarsal (Plate CXXI. fig. 4, iv), though some- 

 what mutilated, exhibits the characteristic modifications of the articular surface in the 

 normal Kangaroos ; and these characters are shown more plainly in the homologous 

 bone of a larger kind of Procoptodon (ib. fig. 8, iv), viz. the non-articular peninsula (f), 

 the backward or plantar production (e, " peroneal process "), with the terminal groove 

 \(g') for the tendon before mentioned ; but this groove is less deep than in the type 

 Kangaroos : the flat surface beneath (fig. 6, h) indicates a larger proportional sesamoid 

 than in the species of Macropus. 



The fore surface of the shaft of the fourth metatarsal (ib. fig. 2) is more even or 

 flattened than in Macropus affirm, Macr. Titan, and the great recent kinds of Kan- 



53* 



