Page:The Osteology of the Reptiles.pdf/29

Rh canal, with the internal opening in front of the external. The internal nares, primitively (Figs. 6, 21, 47) divided by the prevomers and surrounded by the premaxillae, maxillae and palatines, may sometimes (Figs. 49, 71 ) lie between the prevomers and palatines.

The parietal or pineal foramen, very large in certain shell-eating cotylosaurs (Fig. 22), had become inconstant even in that order. It is present, so far as known, in all the Theromorpha (Figs. 33–42), and in the Therapsida (Figs. 43–45), with the exception of a few forms; in the Proganosauria, Ichthyosauria (Fig. 50), Sauropterygia (Fig. 48), the Diaptosauria (Figs. 60–62), and most lizards (Figs. 55, 56), but is absent in some true lizards, the chameleons, and all snakes (Fig. 59). It has been reported in certain doubtful Pseudosuchia and more or less doubtfully in a few phytosaurs and dinosaurs, but with these possible exceptions appears to be absent in all the Archosauria (Figs. 65, 66 , 68, etc.) as also the Chelonia (Figs. 30, 31, 32). Usually located between the parietals anteriorly (Figs. 22, 33, 43, 44, 45, 46, etc.), it may occur between the frontals posteriorly (Fig. 55). In the Anomodontia and Gorgonopsia (Fig. 43) there is a separate bone, the preparietal, a small unpaired element of doubtful homologies, absent in other reptiles, in front of or surrounding the foramen.

The orbits, directed upward sometimes in aquatic animals (Fig. 32), but usually laterally, were primitively (Figs. 2, 3, 22, 23, 33, 43, 44, 65) surrounded by the prefrontal, postfrontal, postorbital, jugal, and lacrimal. The frontal usually forms a part of the upper margin, the maxillae sometimes below (Figs. 30, 48, 49, 55, 56, 59). In snakes (Fig. 59), only the prefrontal and postorbital may be left. Sometimes the postorbital bar is incomplete in lizards (Fig. 56), snakes (Fig. 59), and therapsids (Fig. 45 ). The prefrontal is excluded in some dinosaurs, the postorbital in Araeoscelis (Fig. 52) and Hyperodapedon (Fig. 62 ), leaving not a single element invariably associated with the orbit. Antorbital or preorbital vacuities are very characteristic of the Archosauria, occurring in all phytosaurs (Figs. 66, 67 ) and true pseudosuchians (Fig. 65 ), most Saurischia (Fig. 70 ) and Pterosauria (Fig. 71) and some Crocodilia. Usually there is but one, but there may be two or even three on each side in certain Theropoda (Fig. 70 ).