above, and flattened on the under surface. The whole foot is much compressed laterally, especially at its hinder part, so that the proximal ends of the second and third are thrown behind that of the great fourth metatarsal, and entirely concealed in a view of the dorsal surface of the foot.

The Tree-Kangaroos of New Guinea (Dendrolagus), which habitually live among the boughs of large trees, have

the feet constructed on the same type, but shorter, and more laterally extended. Hypsiprymnodon moschatus, a species of Rat Kangaroo, lately discovered by Ramsay in Northeast Australia, differs from all other members of the family Macropida in the possession of a small prehensile hallux,

without a nail, and presents therefore an approximation to the next group.

In the leaf-eating, climbing Australian Opossums (Phalangista, Fig. 131) and Koalas (Phascolarctos) the second and third toes are also very slender, but the fourth and fifth are more equal, especially in length, the foot is broad, and there is a stronglydeveloped prehensile and opposable, through nailless, hallux.

cb

IV

n

I

FIG. 132.-Bones of right foot of Charopus castanotis (nat. size).

The insect- and root-eating, grounddwelling Bandicoots (Peramelida) differing in many other respects from the Kangaroos, have their hind foot constructed on exactly the same type as in Macropus, even to the relative length of the different digits, though there is often a rudiment of the metatarsal of the hallux. In one remarkable genus (Charopus), already mentioned account of the peculiar structure of the manus (see Fig. 112, p. 310), the same type is carried to a great extreme, the fourth toe (see Fig. 132) remaining of a prodigious size, and the fifth being reduced to even smaller dimensions than the second or third.

on

MONOTREMATA.-In both species the seven usual bones of the tarsus are complete and distinct,' and the five digits have the normal number of phalanges.

1 It has been stated that the cuboid in the Ornithorhynchus is divided into two bones, as in some reptiles, one supporting the fourth and the other the fifth metatarsal; but this is not the case in any specimen which I have examined.

In the Ornithorhynchus the proximal articular surface of the astragalus is divided by a deep groove into two distinct heads, one for the tibia, and the other for the fibula, the latter being the larger of the two; the inner side has a cupshaped socket, into which fits an incurved conical process from the lower end of the tibia. The tuberosity of the calcaneum is broad and bifid at the extremity, and directed not backwards, but towards the tibial side of the foot. In the male there is an additional, large, flat, curved ossicle, on the hinder and tibial side of the plantar aspect of the tarsus, articulated chiefly to the tibia, which supports the peculiar perforated horny spur characteristic of this sex, the function of which has not yet been discovered. There is also a small, rounded, supplementary ossicle, below the tibial edge of the tarsus, near the articulation between the astragalus and scaphoid. The metatarsals increase in length from the first to the fifth. The phalanges are all rather long and slender. The four outer toes are nearly equal; the hallux is somewhat shorter. The ungual phalanges are compressed, slightly curved, and very sharp-pointed.

In the Echidna the astragalus is large, with an irregular, broad, rounded, proximal articular surface, not divided by a groove, and with a much less distinct fossa, for the internal malleolus. The tuber calcanei is directed forwards, it is also bifid, and its external process is much longer than the other and curved towards the plantar surface of the foot. The spur of the male, and the ossicle which supports it, are much smaller than in the Ornithorhynchus. metatarsals are shorter and broader; they increase in length from the first to the fifth. The hallux is very short, and has a flattened, conical, ungual phalanx. The proximal and middle phalanges are all very short and broad. The ungual phalanx of the second digit is extremely long and falcate,

the others gradually diminish to the fifth. The ends of the toes are turned outwards and backwards in the ordinary position of the animal.

The ungual phalanges of both extremities in the Monotremata have a deep median groove, near the base of the under surface, leading at its distal extremity into a foramen.

CHAPTER XX.

THE CORRESPONDENCE BETWEEN THE BONES OF THE ANTERIOR AND POSTERIOR EXTREMITY AND THE MODIFICATIONS OF THE POSITIONS OF THE LIMBS.

THAT a general correspondence exists in the plan of construction of the anterior and posterior extremity cannot fail to strike the most superficial observer, though to follow out this correspondence into all its details has severely exercised the ingenuity of many an anatomist.

It would be quite beside the character of the present work to give an historical account of the numerous and very various views which have been held upon this subject, but I propose to lay before the student in a condensed

1 For the bibliography of this question, see Ch. Martins, "Nouvelle comparaison des membres," (Mem. de l'Academie de Montpellier, 1857); Mivart, "On some Points in the Anatomy of Echidna hystrix," (Linn. Soc. Trans. xxv. 1866); and Rolleston, "On the Homologies of certain Muscles connected with the Shoulder Joint," (ibid. xxvi. 1869). See also Humphry's "Observations in Myology," 1872; A. Goette, "Ueber Entwickelung und Regeneration des Gliedmassenskelets der Molche," (Leipzig, 1879); H. Strasser, "Entwickelung der Extremitaten-knorpel bei Salamandern und Tritonen," (Morphol. Jahrb. 1879); A. Sabatier, "Comparaison des Ceintures et des