OF THE AUSTRALIAN MARSUPIALIA. 177 



MACROPODID^. 



The general type of pes in the Macropodidse represents a terrestrial derivative of the 

 second arboreal phase. The tendency is towards digitigrade modification and functional 

 monodactylism. There is an almost perfect parallelism between this family and the 

 Peramelidoo *, although the two are easily proved to be of totally independent derivation. 



The pes of Uijpsipivjmnodon (PI. 7. fig. 21) is familiar as presenting an intermediate 

 condition between the general type represented I\y the arboreal Phalangeridae and that 

 characteristic of the Macropodidse. The whole foot is comparatively short. The 

 hallux is fairly well developed and possesses the full number of phalanges. It is 

 opposable, although not to the same extent as in the Phalangeridge. This condition 

 contrasts strongly with that in tlie remaining Macropodidfe, where the hallux is in- 

 variably absent. The plantar surface presents the full number of transversely striated 

 pads. The first and second digital ones tend to fuse together in connection with the 

 incipient narrowing of the foot. 



Contrary to what might be expected from their dental relations, Hypsiprtjmnodon 

 shows no points of special affinity in its foot-structure with Bettongia and jEpyprymnus. 

 In both of the latter the pes is already well specialized ; it is very narrow and gi-eatly 

 elongated, much more so, in fact, than in many otherwise more specialized forms. There 

 are no indications of plantar pads, with the exception of a large elevation at the base of 

 the functional digits. The various species show comparatively little variation. 



In Foto7'Ous (PI. 7. fig. 23) we find a fairly close approximation to Hi/psiprymnodon. 

 The pes is much shorter than in the preceding genera, and the digits show the lesser 

 degree of elaboration and also the freedom of lateral movement characteristic of Hypsl- 

 pryonnodon and the Phalangeridae. The full number of plantar pads has been retained, 

 but their transverse striations are broken by irregular longitudinal markings. The 

 hallucal and marginal pads are in process of reduction, as in the Peramelidte. The first 

 and second digital pads are partially joined together, as in Hypsiprymnodon. So far as 

 can be judged from dried specimens, there is little variation in the different species. 

 Caloprymnus shows a much closer approximation in its foot-structure to Bettoiig/a and 

 JEpyprymnus than to Putoroiis, as seen in the greater elongation, the greater elaboration 

 of the fourth digit, and the absence of plantar pads, with the exception of the large 

 basal structure described for the former genera. The length of the pes as compared 

 with the breadth is greater in Caloprymnus than in BettoHgia, with the exception of 

 B. Gaimardl, and almost twice as great as in Potorous. These relations appear at first 

 sight to oppose the serial arrangement arrived at from a study of the dentition ; but 

 there is no doubt that the dental relations are of a more fundamental character. The 

 fact that the foot-structure of the Plialangerinoe is practically homogeneous tliroughout 

 shows that the greater resemblance of Hypsiprymnodon to Fotorons means only 



* The arrangement of the tarsals and metatarsals furnishes a point of distinction. In the Pcramelidic the 

 enlarged fourth metatarsal is supported in part by tlic ectoouneiform and in part by the cuboid, while in the 

 Macropodidse it is practically supported by the culioid alone. 



