The alligator and its allies

by A. M. Reese

The ischium (2) is a slightly arched bone, its ventral end a flattened blade articulating with its fellow, its dorsal end enlarged and thickened to articulate with the ilium, pubis, and epipubis. This dorsal end, which forms the ventral side of the acetabulum, is divided into two distinct articular surfaces by a deep, rounded notch; the posterior and larger surface articulates with the ilium, the anterior surface about equally with the pubis and epipubis.

The pubis (3), which is much the smallest element of the pelvis, is a small mass of cartilage lying between the ilium above and the ischium below. It forms a small part of the anterior wall of the acetabulum.

The epipubis (4) is a slightly arched bone, somewhat enlarged at its proximal end where it unites with the ischium, and flattened out into a fan-shaped extremity, where it is united with its fellow and with the last pair of abdominal ribs by the broad, thin sheet of cartilage or fibrous tissue noted in connection with the abdominal ribs. As mentioned above, it is called by Wiedersheim and others the pubis. Near the center of the acetabulum there is a small foramen.

The posterior limb ([Fig. 29]) consists of the usual divisions—thigh, shin, and foot. The femur (f) is a bone of the same general outline as the humerus, though slightly longer and heavier. The head, for articulation with the acetabulum, is rather hemi-elliptical than hemispherical in shape, the long axis of the ellipse being vertical. The distal enlargement is of at least as great, if not greater, bulk than the proximal and shows some indication of a division into two articular surfaces. The ventral side of the femur near the proximal end shows a fairly distinct trochantal ridge.

The shin or crus is made up of two well-developed bones, the tibia (t) and fibula (fb), the former being somewhat longer and considerably thicker than the latter.

Fig. 29. Posterior Limb.

ca, calcaneum or fibulare; cl, claw; f, femur; fb, fibula; t, tibia; t³, t⁴⁻⁵, tarsalia; tb, tb′, tibiale-centrale; IV, V, 4th and 5th metatarsals.

The tibia consists of a cylindrical shaft with enlargements of about equal size at the ends. The proximal end forms most of the knee joint, the distal end articulates with a tarsal element said by Reynolds to represent the fused astragalus and centrale, by Wiedersheim called the astragalus, and said to represent the united tibiale, intermedium, and centrale (tb, tb′). The fibula articulates by a small enlargement at its proximal end with the femur, and by an enlargement of about equal size, at its distal end, with the fibulare or calcaneum (ca), and with a small facet on the above-mentioned tibiale-centrale element.

The tarsus is much modified and consists of four elements, in two rows; those of the proximal row are much larger than the two distal elements. Articulating with both tibia and fibula, as mentioned above, and with the first metatarsal and one of the distal tarsalia, is the large and irregular tibiale-centrale element of Reynolds (tb, tb′). In the tarsus here shown it consists of two elements. Post-axial in position is the calcaneum or fibulare (ca), articulating with the preceding tarsal element, with the fibula, with the rudimentary fifth metatarsal, and with the distal tarsal element said by Reynolds to represent the fourth and fifth tarsalia. The calcaneum is extended caudad into a prominent knob quite like the heel of the higher mammals.