Page:The Osteology of the Reptiles.pdf/139

Rh hundred in number. Anteriorly they are covered or underlaid by the distal end of the interclavicle. In the modern Sphenodon (Fig. 94 ) there are about twenty-four such rods, each composed of a median, unpaired piece and a lateral splint, every alternate one of the first eleven attached to the end of a dorsal rib. In the Phytosauria they are similar, nineteen or twenty in number. The Choristodera (Fig. 94 ), Plesiosauria, and Ichthyosauria, aquatic reptiles, have larger and stouter parasternals, consisting of a straight or slightly curved median piece, and three or four lateral splints on each side. The Crocodilia (Fig. 121 ) have seven or eight pairs, each composed of two slender rods on each side (not joined in the middle). In earlier members of the order there was a larger number, and some of them, at least, were composed of the usual V-shaped median piece and a lateral splint on each side. The last pair is enclosed in a dense sheath of fascia continuous with the ends of the so-called pubes.

Among the modern lizards abdominal ribs are often present, especially in the chameleons, each composed of one broadly V-shaped piece, either connected with the dorsal ribs or free, sometimes paired and usually cartilaginous. Only in a few forms have they been observed as slender ossifications. Clearly endoskeletal in origin, they have been supposed to be not true parasternals, and have been called distinctively abdominal ribs. That they are not continuations of the dorsal ribs seems evident from the fact that they are sometimes much more numerous than the overlying vertebrae. These lacertilian ribs are located, it is said, in the rectus abdominis muscles. The parasternals of Sphenodon are in the superficial part of the rectus and external oblique muscles, and are united by a dense sheath of fascia.

The later pterodactyls have five or six flattened parasternals, the anterior ones broadly V-shaped, the posterior ones paired. In the earlier pterodactyls the unpaired median piece has one or two lateral splints. They have also been observed in various genera of theropod dinosaurs. In the Chelonia they are represented by the posterior three pairs of plastral elements, as usually accepted, but it is possible that these are really dermal elements and [not] true parasternals. The extinct Saphaeosaurus (Sauranodon) had a full armature of ossified parasternals similar to those of Sphenodon.

Parasternal ribs have long been considered to be of dermal origin,