Page:The Osteology of the Reptiles.pdf/31

Rh (Fig. 21 ), and Theromorpha (Fig. 45 ), and is generally present in later reptiles though absent or vestigial in the Crocodilia.

The cranial region thus exposed by these various openings has been exposed to a greater or less degree in most Chelonia (Figs. 30–32) in a different way: by the emargination of the roof bones from behind or from behind and below, until, in some forms like the terepenes, the whole temporal roof is lost.

Posterior palatine or suborbital openings occur in most reptiles (Figs. 55, 63, 66, 69, 72) since the Theromorpha, but are absent in some turtles. They are situated between the palatines and maxillae posteriorly, and are usually also bounded in part by the ectopterygoids. They do not occur in the Cotylosauria (Figs. 6, 21, 24, 29) or Theromorpha (Figs. 40 , 42 ), though present in many Therapsida.

The primitive relations of the skull elements may be discussed seriatim, with their chief modifications in later reptiles.

Premaxillae (px). Primitively short (Figs. 2, 3, 4, 19, 22, 33, 43), articulating with maxillae, nasals and prevomers, the posterior process forming a partial division between the nasal chambers. They form the anterior boundary of the external and internal nares. Four or five teeth in each.

Elongate in the strictly aquatic reptiles (Figs. 46, 47) and in the Pterosauria. In the plesiosaurs (Figs. 46, 47), pterosaurs (Fig. 72 ), some lizards (Fig. 56), and thalattosaurs (Fig. 61), a median prolongation separates the nasals, articulating directly with the frontals, in the first group (Fig. 46) sometimes directly with the parietals, separating the frontals. Edentulous in the chameleon lizards (Fig. 55), they take no part in the boundary of the nares. They are also edentulous in the turtles (Figs. 30–32), anomodonts (Fig. 44 ), some dromasaurians, the later pterodactyls (Fig. 72), most ornithischians (Fig. 70 ), the chameleon lizards, and many snakes. Teeth, when present, are in a single row and rarely exceed five or six in number in each, though there are as many as twenty-three in some phytosaurs (Fig. 66) and even more in ichthyosaurs, where the dentigerous border is greatly elongated. The dentigerous part is short in the long-faced plesiosaurs (Fig. 48 ). They are often fused (Fig. 72), and sometimes united with the nasals (Fig. 54 ).