Page:The Osteology of the Reptiles.pdf/296



temporal openings, the upper bounded by the parietal above, the postorbito-squamosal arch below; the lateral by the postorbito-squamosal arch above, the jugal, or jugal and quadratojugal, below. A single coracoid on each side; no cleithra. Pelvis with pubo-ischiatic opening. Quadrate fixed or partly movable, never streptostylic.

The phyletic unity of this great division of reptiles and their descendants, the birds, admits of little or no doubt. In much probability they were derived from the single-arched type with the lateral opening, by the simple separation of the postorbital and squamosal from the parietal. Until recently it was confidently believed that the most primitive and oldest representative of the subclass was Paleohatteria, from the Lower Permian. In all probability, if not certainty, this form did not have the upper temporal opening, and must therefore be included in the more primitive group, the Theromorpha. At present the oldest known form referable to the subclass is Youngina, from the Upper Permian of Africa, an intermediate type peculiar in its retention of certain skull bones lost in all other members. It is, however, yet very imperfectly known. Doubtless many other forms from the Permian with these and yet other primitive characters await discovery.

Skull elongate, with palatal teeth; an antorbital vacuity. Skull only known.

[Triassic. Proterosuchus Broom, South Africa.]

. Skull with interparietals and tabulars (? supratemporals). Skull short; no antorbital vacuity ; probably with palatal teeth. Skeleton otherwise unknown.

Upper Permian. Youngina Broom, South Africa.