bone, and in an adult Fin Whale (Balanoptera musculus), sixty-seven feet long, this was found to be only represented by an oval nodule of cartilage about the size of a walnut.1 Even this is wanting in some species of the group, as B. rostrata and B. borealis.

No trace of any structure representing the skeleton of the hind limb, beyond the pelvis, has yet been detected in any of the Odontocetes.

In none of the existing SIRENIA are there any rudiments of the hind limb proper, but the extinct Halitherium had an ossified femur, articulated to a well-defined acetabulum in the pelvis.

In the terrestrial and fossorial EDENTATA the femur is generally short and broad. There is a third trochanter in the Armadillos and Orycteropus, and a sharp ridge along the whole external border in Myrmecophaga. The fibula is as long as the tibia. In the Armadillos these bones are commonly ankylosed together at each extremity, but curve away from each other at the middle, leaving a wide interosseous space. In the Anteaters they are both nearly

straight and parallel.

In the Sloths the femur is long, slender, and flattened from before backwards. There is no third trochanter; the head is large and globular, and placed near the middle of the proximal end of the shaft, with the axis of which it more nearly coincides than in most Mammals. The tibia and fibula are complete, and more nearly equal in size than in most Mammals. They are both curved, so as to be separated considerably in the middle part of the leg. The lower end of the fibula has a conical prominence which turns inwards, and fits into a depression on the outer side

1 "Proceedings of the Zoological Society, 1865," p. 704.

of the articular surface of the astragalus, as a pivot into

a socket.

In none of the MARSUPIALIA is a third trochanter present on the femur. The fibula is always well developed, and its proximal extremity is often produced into a wellmarked process, to the top of which a sesamoid bone is not

FIG. 121.-Anterior aspect of bones of right leg of Ornithorhynchus anatinus, *. ffemur; tibia; ƒ fibula; p patella.

unfrequently attached; but, on the other hand, the patella, except in the Peramelida, is unossified or quite rudimentary. In Charopus and Hypsiprymnus the distal end of the fibula is united with the tibia. In the climbing Australian Phalangers and Koalas, which have broad hind feet, with an opposable hallux, there is a greater freedom of movement between the fibula and tibia than in other Mammals,

approaching in some degree to the rotation often permitted between the radius and ulna.

In the MONOTREMATA the femur (Fig. 121, f) is of very remarkable form, being short, flattened from before backwards, narrow in the middle of the shaft, and very broad at each end; the trochanters are both well developed, and the head placed between them on a very short neck, and with its axis directed quite towards the anterior or dorsal aspect of the bone. The fibula (ƒ') is large and straight; it has a broad flattened process, completed by an epiphysis, projecting from the upper extremity above the point of articulation with the tibia, much resembling the olecranon of the fore limb. The tibia (†) is strongly curved in Ornithorhynchus, but straight in Echidna. In both genera the patella (p) is well developed.

CHAPTER XIX.

THE HIND FOOT OR PES.

THE terminal segment of the hind limb is the foot or pes. Its skeleton presents in many particulars a close resemblance to that of the manus, being divisible into three parts: (1) a group of short, more or less rounded or squareshaped bones, constituting the tarsus; (2) a series of long bones placed side by side, forming the metatarsus; and (3) the phalanges of the digits or toes (see Fig. 122, p. 341).

The metatarsal bones never exceed five in number, and the phalanges follow the same numerical rule as in the manus, never exceeding three in each digit. Moreover, the first digit (counting from the tibial side), or hallux, resembles the pollex of the hand in always having one segment less than the other digits. If one toe only is absent, it is the first or hallux. Unlike those of the manus, any of the five toes of the foot may be absent or at least rudimentary; the fourth and fifth are best developed in Macropus, the third and fourth in Pecora, the fourth alone in Charopus, and the third alone in Equus.

The bones of the tarsus in many of the lower Vertebrata closely resemble both in number and arrangement those of

the carpus, as shown in Fig. 90, p. 281. They have been described in their most generalised condition by Gegenbaur under the names expressed in the first column of the following table.1 The names in the second column are those by which they are most generally known in this country, and which will be used in the present work, while in the third column some synonyms, occasionally employed, are added.2

The bones of the tarsus of Mammals present fewer diversities of number and arrangement than those of the carpus.

1 "Untersuchungen zur vergleichenden Anatomie; Carpus und Tarsus."

1864.

2 Recent researches of Baur, "On the Morphology of the Tarsus in the Mammals," American Naturalist, January, 1885, make it probable that a certain bone on the tibial side of the tarsus of Hyrax, many Edentates, Ornithorhynchus and Rodents (cf. p. 349) and hitherto looked upon as a sesamoid bone-is the rudimentary tarsale tibiale, whilst the astragalus is the intermedium, representing the lunare of the hand. This tibiale is frequently fused with the centrale, the navicular bone in such cases containing the elements of the centrale and tibiale. The homologies of the proximal row of the Mammalian tarsus would therefore, according to Baur, be the following-

See also Bardeleben, “Zur Morphogie des Hand- und Fuss-skelets." Sitzungsberichte Iena. Gesellsch. Med. Nat. 1885.