152 



VERTEBRATE SKELETON 



less pronounced basipterygoid process of the basisphenoid. Except 

 in Amphisbaenans and Chameleo (fig. 165) epipterygoid and trans- 

 versum are present. The basisphenoid is continued forwards by 

 a longer or shorter rostrum, apparently parasphenoidal. The bones 

 in the lower jaw are numerous except in Ophidia where there is some 

 fusioli. 



The chondrocranium is best known in lizards. In the early stages (fig. 161) 

 only parachordals, trabeculae and sphenolateral parts are present in front and 

 there are four incomplete arches in the postotic regions, these separating the 

 roots of the twelfth nerve — the same number as in Mammals and three more 

 than in Amphibia. The basal plate is completed later (fig. 162), the parachor- 

 dals meeting ventral to the chorda, the posterior part of each side arising as 



Fig. 161. — Reconstruction of chondrocranium of Ascalabotes (SewertzofT, 'oo). 

 a, sphenolateral; e, eye; h, hypophysis; ms, mesencephalon; ov, otic vesicle; n, notochord; 

 p, parachordal plate; Ir, trabecula; v, second occipital vertebra; i, first head somite. 



a pillar (vertebral arch) with the ninth nerve between it and the otic capsule, 

 the tenth and eleventh nerves behind it. The posterior edge of the basal plate 

 is excavate, separating a pair of occipital condyles, the excavation disappearing 

 later, resulting in the single condyle of the adult. Chondrification of each otic 

 capsule begins in the floor, the median wall being last to develop, while, to 

 accommodate the large lagena, the epithelial parts of the ear invade the basal 

 plate. The connexion of the two capsules by the synotic tectum is later. The 

 tectum, arising from both capsules and occipital vertebrae, bears a median 

 anterior process as in Anura. 



At first each trabecula is a separate bar at a strong angle with the posterior 

 part of the cranium. Later the two extend forwards and fuse as the trabecula 

 communis, uniting behind with the basal plate. The common trabecula expands 

 dorsally as the interorbital septum, dividing dorsally into the supraseptal plates, 

 each usually with a septal fenestra. Each trabecula gives off laterally behind, 

 a basipterygoid process, apparently arising from pterygoid tissue, the rest of 

 which forms the pterygoquadrate. 



The relations of the sphenolateral parts to later conditions are uncertain. 

 In Laccrta (fig. 144) there is a marginal taenia which joins the roof of the oric 



