THE RIBS AND STERNUM I2T 



hundred in number. Anteriorly they are covered or underlaid by the 

 distal end of the interclavicle. In the modern Sphenodon (Fig. 94 b) 

 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 c), 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 

 Crocodiha (Fig. 121 c) 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, espe- 

 cially 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 ob- 

 served 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 some- 

 times 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 possi- 

 ble that these are really dermal elements and [not] true parasternals. 

 The extinct Saphaeosaurus (Sauranodon) had a full armature of ossi- 

 fied parasternals similar to those of Sphenodon. 



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



