290 PROSAURIA CHAP. 



Order II. PROSAURI. 



Mostly extinct, chieily Permian and Triassic, terrestrial, un- 

 armoured reptiles with deeply biconcave vertebrae, numerous 

 intercentra and chevron-bones, fixed quadrates, complete penta- 

 dactyle limbs and shoulder - girdle, entepicondylar foramina, 

 acrodont teeth, and many small abdominal ossifications. 



The Prosauri differ from the Microsauri, with which they 

 are closely allied, by the more advanced solidification of the 

 vertebrae, the reduction of the tubercular portions of the ribs, 

 the presence of an entepicondylar foramen in the humerus, and the 

 loss of the dermal ossifications on the upper surface. 



Their ancestors are the Microsauri, whilst they themselves 

 seem to be very near the root whence have sprung most, if not 

 all, other main branches of the reptiles, notably Crocodilia, Dino- 

 sauria, and Sauria. In fact the Prosauri, although apparently 

 few in number, seem to represent the central stem of the 

 reptilian tree. Only one of them is still surviving, the famous 

 Splienodon, now represented by a single species in Xew Zealand. 



Sub-Order 1. ProtorosaurL — The ventral half of the pelvis 

 seems to have formed one broad, continuous mass of cartilage in 

 which the pubic bones are represented by a pair of oval, rather 

 disc-shaped ossifications, while the ischia are more elongated. 

 The pelvis consequently still bears a great resemblance to that of 

 the Microsauri, and therel)y also to the Stegocephalous condition, 

 but the ilium seems to be attached to more than two vertebrae. 

 The vertebrae are deeply biconcave, perhaps even with a persistent 

 continuous chorda. The neural arches bear high, laterally com- 

 pressed spines, but no diapophysial or lateral processes, the ril)s 

 being placed mostly intervertebrally and having lost their tuber- 

 cular portions. The ribs are continued to about the sixth 

 caudal vertebra. Intercentral w^edges exist in an unbroken series 

 between all the vertebrae from the atlas to the tail, where they 

 are represented by movable chevrons. A costal sternum seems 

 to be absent, unless it was quite cartilaginous. The shoulder- 

 girdle is complete, consisting of a long interclavicle, clavicles, 

 disc-shaped coracoids, and scapulae ; but there are no cleithra, and 

 no indication of precoracoids or even notches in the coracoids. The 

 fore- and hiiid-]iiiil)S are complete and primitive, with five digits. 

 The abdomen is protected by numerous oat-shaped little ossifica- 



