SKULL — AMPHIBIA 1 23 



while supra temporal and squamosal cover the space between parietal 

 and quadrate. The other bones of the roof need no mention other 

 than that on page 120. 



The ventral side of the cranium (fig. 129) has at least a pair of 

 large fenestrae, bounded laterally by the anterior end of the ptery- 

 goids, medially by the wall of the cranial cavity. The vomers are 

 usually large, and the parasphenoid extends from them to the base 

 of the skull. All of these bones may bear teeth. A few genera have 

 an OS transversum intercalated, as in many reptiles, between ptery- 

 goid and maxilla. In the base of the skull the basioccipital is often 

 excluded from the foramen magnum; apparently a supraoccipital 

 never ossifies. 



Some genera have the number of bones of the lower jaw reduced 

 as in recent Amphibia; others (lig. 130) have the whole series of the 



Fig. 130. — Medial side of lower jaw of Trimerorachus (Broom, '13). an, angulare; 

 ar. articulare; d, dentale; co, coronoid; g, goniale; pa, preangular of Broom; sa, surangu- 

 lare; sp, splenial. 



more primitive reptiles and fishes. Sometimes (Archegosaurus, 

 Branchiosaurus) the young, as shown by the fossils, had well- 

 developed branchial arches, some of which are described as bearing 

 teeth, probably similar in function to the gill-strainers (p. 115) of 

 many fishes. 



Urodela, whether primitive or neotenic, are most like the Stego- 

 cephals in cranial structure, but with a greatly reduced number of 

 dermal bones, the loss being greatest in forms like Necturus and 

 Proteus (fig. 134) which have resumed a purely aquatic life. The 

 Urodele chondrocranium has been studied in several species and the 

 following outline gives its sahent points, features common to all 

 Amphibia (p. tio) being largely omitted. 



The early formed basal plate includes the centrum of an occipital vertebra 

 which at first is separate from the rest of the cranium. The plate is fenestrate 

 later by resorption of cartilage. In Necturus the sphenolateral cartilage arises 

 apart from the trabecula, fusing with it later and forming the trabecular crest. 

 The otic capsules arise independently of the basal plate, joining it later, the 

 seventh nerve passing through or beneath the floor of the capsule; the tenth 



