There is a sesamoid bone at the palmar aspect of each of the distal articulations of the phalanges in the Echidna ( fig. 175), and at all the digital articulations in the Ornithorhynchus (fig. 173, u,d).
The ungual phalanges are long, depressed, nearly straight, of great strength in the Echidna, in which each of them is perforated at the palmar aspect (fig. 175).
Of the pelvic pelvis of the Monotremes bears a resemblance to that of Reptiles in the length of time during which the three components of each os innominatum remain distinct, especially in the Echidna; and in the great development of the ilio-pectineal spine, which equals in size that of the tortoise, in the Ornithorhynchus ; the pelvis of the Echidna resembles that of Birds in the perfo ration of the acetabulum (fig. 177, g), but the pelvis in both Monotremes chiefly resembles that of the higher implacental Mammalia in the presence of the marsupial bones.
The ilium (fig. 177, a) is a short, strong, trihedral bone, with the upper extremity ex panded and everted in the Ornithorhynchus. The ischium (b) has its tuberosity prolonged backwards in an obtusely-pointed form in the Ornithorhynchus. The pubis in the same animal, besides having the spinous process directed forwards, gives off a second smaller process, which projects outwards; this process is present, but less developed in the Echidna (fig. 177,f). The pubis (c) and ischium con tribute an equal share to the formation of the foramen obturatorium (h) and to the symphysis which closes the pelvis below: the symphysis is relatively shorter in the Echidna (d) than in the Ornithorhynchus.
The marsupial bones (fig. 173, A, X x, 177, e) are relatively larger and stronger in the Mono tromes than in the ordinary Marsupialia, the Koala excepted; their base extends along the anterior margin of the pubis from the symphysis outwards to that of the spinous process (fig. 177, f); they are relatively longer in the Echidna (c) than in the Ornithorhynchus; they always remain moveably articulated with the brim of the pelvis.
is short, broad, and flattened ; its head rises, like that of the humerus, from the middle of a broad expanded proximal end, having on each side a strong process, the outer one representing the great, the inner one the small trochanter. In the Echidna a project ing ridge extends from the great or outer tro chanter beyond the middle of the bone ; the whole of the inner part of the shaft is bounded by a trenchant edge ; both outer and inner margins of the bone are trenchant in the Ornithorhynchus. The distal end of the femur
is expanded transversely, but compressed from before backwards. The rotular trochlea is flat transversely, convex vertically, in the Echidna; it is hardly definable when the cartilage is separated from the bone ; but the patella itself is well developed, and ossified in both Mono tremes (fig. 173, A, p).
The tibia is straight in the Echidna, but bent, with the convexity next the fibula, in the Ornithorhynchus; its cristre are slightly marked.
The fibula is slightly bent in the Echidna, but is straight in the Ornithorhynchus ; in both Monotremes it is longer than the tibia by the extent of a process which rises upwards beyond the proximal articulation of the fibula, and most strongly expresses the analogy of this bone with the ulna: this process (fig.173, v) reaches half way up the back of the femur in the Ornithorhynchus, and, like the olecranon, is greatly expanded at its termination. Cuvier* indicates the resemblance of this structure in the Monotremes with the fibula and the super numerary bone imposed upon its enlarged prox imal end in the pedimanous Marsupials.
The tarsus (figs. 178, 179) consists of a scaphoid (a), astragalus (b), a calcaneum (c), three cuneiform bones (d,e,f), and a cuboid (g) in the Echidna; but the cuboid in the Ornitho rhynchus is divided into two bones, as in some Reptiles, one for the fourth and the other for the fifth metatarsal bones. In both Mono tremes there is a sesamoid hone (fig. 178, l) placed at the interspace between the astragalus and the naviculare; a second supernumerary bone (*) is articulated to the posterior part of the astragalus, and supports the perforated spur which characterizes the male sex (fig. 173, x, d).
The calcaneum of the Ornithorhynchus ter minates by sending outwards a short obtuse tuberosity ; in the F,chidna this part is more slender, and is singularly directed inwards and forwards, nearly in a line with the digits (fig. 179, e).
The astragalus in the Ornithorhynchus pre sents a double trochlea above for the tibia and fibula, and a depres sion on its inner side, which receives the in curved malleolus of the tibia, almost as in the Sloths. The toes have the same number of bones as in other Mam malia ; their size and form are more alike in the two Monotrema tous genera than those of the fingers: the un gual phalanges, like the claws they support, are more curved than those on the fore foot, but like them they are perforated on their in ner and concave side (fig.179).