In _Sphenodon_, then, on the dorsal surface of the skull, are the large =supratemporal fossae= (fig. 52, 20). Their inner margins are separated from one another by the parietal walls of the cranium, while externally each is bounded by a bony arch, the =supratemporal arcade=, formed of the postfrontal, postorbital, and squamosal. Posteriorly the boundary is formed by a =post-temporal bar=, formed by the parietal and squamosal. Below the supratemporal arcade is another large vacuity, the =infratemporal= or =lateral temporal fossa= (fig. 52, 21). This is bounded above by the supratemporal arcade, and is separated from the orbit in front by the =postorbital bar=, formed by the union of outgrowths from the jugals and postorbitals. Behind it is bounded by a continuation of the post-temporal bar formed of the squamosal and quadratojugal, and below by an =infratemporal arcade=, which is chiefly composed of the quadratojugal and jugal. Below the post-temporal bar is a third vacuity, the =post-temporal fossa= (fig. 52, D, 23), bounded above by the post-temporal bar and below by the exoccipital and opisthotic. _Sphenodon_ and the Crocodilia are the only living reptiles with complete supratemporal and infratemporal arcades, but they are both present in the extinct Pterosauria and some Dinosauria. Supratemporal fossae, bounded below by supratemporal arcades, occur in all reptiles except some Chelonia, the Ophidia, the Geckonidae among Lacertilia, and the Pariasauria and others among Theromorpha; they are specially large in _Nothosaurus_ among the Sauropterygia, _Dicynodon_ among the Theromorpha, and many Crocodilia and Pterosauria. In some Dinosaurs, such as _Ceratosaurus_, they are very small, while the infratemporal fossae are correspondingly large. In _Elginia_[93] (Theromorpha) and some Chelonia, such as _Chelone_, there are no fossae on the surface of the skull, a complete false roof being developed; in other Chelonia, such as _Trionyx_, the true cranium is freely visible, the only part of the false roof developed being the infratemporal arcade. In many reptiles large =pre-orbital vacuities= occur; they are specially large in the Pterosauria and in some of the Crocodilia and Dinosauria (fig. 35, 3). In some Pterosauria they are confluent with the orbits. The premaxillae are usually separate, but sometimes, as in some Ophidia (fig. 51, 1), Chelonia, Lacertilia (Agamidae), and Dinosaurs (Ceratopsia) they are united. In the Dinosaur _Hadrosaurus_ they are exceedingly large and spatulate. In the Rhynchocephalian _Hyperodapedon_ they are drawn out into a strongly curved beak.