Fig. 21.—Anterior extremity of Ophiacodon.
The radius and ulna are always distinct bones in reptiles, and always freely movable on each other; they are usually shorter than the humerus, but in some springing and climbing reptiles they are quite as long.
The carpus or wrist of reptiles consists primitively of eleven distinct, irregularly shaped bones, which articulate more or less closely with each other in three rows. Those of the first row, all true carpals, are known usually as the radiale, intermedium, ulnare, and pisiform, corresponding quite with the bones of the human wrist known as the scaphoid, lunar, cuneiform, and pisiform. The second row has but two bones, on the radial side, known as the centralia; while the third row has a bone to correspond to each of the metacarpals, five in number, and collectively known as the carpalia. Some or indeed all of these bones may be either absent or unossified, that is, remaining through life as nodules of cartilage. Seldom, however, are there less than nine bones in the carpus of reptiles.
The metacarpals, like the digits, primitively were five in number, and seldom are there less, though the fifth is sometimes lost, and rarely also the first. They are more or less elongate bones, increasing in length from the first to the fourth, with the fifth usually shorter. The first and the fifth are usually more freely movable on the wrist than are the other three.
The number of joints or phalanges in the fingers of all primitive reptiles is that of the modern lizards and the tuatera, that is, two on the first finger or thumb, three on the second, four on the third, five on the fourth, and three on the fifth. The crocodiles have one less phalange on the fourth digit; the turtles have usually two less on the fourth and one less on the third, that is, with precisely the same arrangement that is found in our own fingers and that of mammals in general, two on the thumb and three on each of the other fingers. As exceptions the river turtles have four bones in the fourth digit. And this mammal-like and turtle-like arrangement of the phalanges was that of those early reptiles, the Theriodontia, from which the mammals arose. The last or ungual phalange of reptiles is usually claw-like, that is, sharp, curved, and pointed, but sometimes it is more nail-or hoof-like.
PELVIC OR HIP GIRDLE
The pelvic girdle or pelvis in reptiles and higher animals consists of three bones on each side, often closely fused in adult reptiles and together known as the innominate bone. The upper or dorsal one of these three bones—that to which the sacrum is attached—is the ilium; the one on the lower or ventral side in front is the pubis; and that on the ventral side behind is the ischium. On the outer side, where these three bones meet, is a cup-like depression, sometimes a hole, called the acetabulum, for the articulation of the head of the thigh bone, homologous with the glenoid articulation of the pectoral girdle, which, as we have seen, was originally formed by three bones, the scapula, coracoid, and metacoracoid, the two latter bones, like the pubis and ischium, meeting in the middle line below. In all the primitive and early reptiles the pubis and ischium form a continuous plate of bone without holes in it, except a small one just below the acetabulum in the pubis, called the obturator foramen, and corresponding to the supracoracoid foramen of the coracoid. One may almost always recognize these two bones by the presence of the foramen. This “plate-like” condition of the pelvis has been lost in all late and modern reptiles by the appearance of a larger or smaller vacuity between the pubis and ischium, either paired, when it corresponds quite with the so-called obturator opening of mammals, or singly in the middle. This old-fashioned character, like the old-fashioned type of pectoral girdle, disappeared entirely about the close of the Mesozoic period, the Choristodera, described in the following pages, being the last of the kind.
