Page:The Osteology of the Reptiles.pdf/173



pairs of limbs are almost always present in reptiles, composed, as in mammals, of four analogous segments: the arm and thigh bones, conveniently called propodials; the forearm and leg bones, or epipodials; the wrist and ankle bones, or mesopodials; the manus and pes, composed of metacarpals and metatarsals, or metapodials; and a variable number of finger and toe bones, known as phalanges.

The limbs are best understood and described as though directed outward from the long axis of the body (Fig. 128), the palms of the hands and soles of the feet turned downward or to the ventral side, the epipodials parallel, the thumb or pollex, and the big toe or hallux, on the anterior or preaxial side, the little finger and little toe on the posterior or postaxial side. The terms outer and inner are often applied to the anterior extremity, as though directed backward in the axis of the body, the thumb on the outer side. The hind extremities are sometimes described as though parallel with the long axis of the body, with the big toe on the inner side. As the hallux is analogous with the pollex, this nomenclature places them on the opposite sides and should not be used for any vertebrates.

The fore and hind limbs of terrestrial reptiles are of approximately equal length, the hind pair the longer. In aquatic reptiles [e.g., ichthyosaurs, mosasaurs] the front pair are often the larger, and usually the longer; in volant reptiles [pterosaurs] they are much longer than the hind pair. In bipedal reptiles [e.g., later Theropoda] or those usually assuming this posture in locomotion, they are smaller or very much smaller. In climbing and cursorial reptiles the limbs are more or less, sometimes very much, elongated and slender (Fig. 155). The digits of fleet, crawling reptiles are long; those of the more upright-walking kinds (Figs. 145, 141 ), in which the digits of the two sides are brought more nearly parallel to each other, are short. The articular surfaces of the limb joints of aquatic reptiles (Figs. 149, 158) are poorly developed, unextensive, and more or less cartilaginous.