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, Dinosauria, 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 _Sphenodon_, now represented by a single species in New Zealand. SUB-ORDER 1. PROTOROSAURI.–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 thereby 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 compressed spines, but no diapophysial or lateral processes, the ribs being placed mostly intervertebrally and having lost their tubercular portions. The ribs are continued to about the sixth caudal vertebra. Intercentral wedges 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 hind-limbs are complete and primitive, with five digits. The abdomen is protected by numerous oat-shaped little {291}ossifications, which are arranged in many transverse or rather chevron-shaped rows, still greatly resembling the condition prevailing in the Microsauri, except that they have sunk deeper into the skin, being no longer directly covered by the scales. The skull, being no longer completely encased by bones, and possessing now wide supra- and infra-temporal fossae, appears at first sight much like that of a generalised lizard, except that it possesses three very conspicuous and distinct arcades in the temporal region: namely, the orbito-squamosal bridge across the temporal fossa, formed by the postorbital and squamosal; the arch formed by the squamosal with the postero-lateral buttress of the parietal; and the infratemporal arch or jugal bridge. The jugal itself is long, connecting the quadrato-jugal with the maxillary and lacrymal, and sending up an ascending process to the