THE LIZARD. 135 



of cartilage, with a small central fontanelle, and produced 

 posteriorly into two slender flattened cornua (st 1 ) : with the 

 antero-lateral edges of the sternum the shoulder-girdle 

 articulates ( 84), to its postero-lateral edges and cornua the 

 sternal ribs are united ( 23). 



23. The five anterior thoracic ribs : each consists of a 

 dorsal bony portion (vertebral rib) articulated to the 

 corresponding vertebra by an undivided head, and of a 

 ventral cartilaginous portion (sternal rib) attached to the 

 sternum. The first three sternal ribs are united with the 

 postero-lateral edges of the sternum, the fourth and fifth 

 with its cornua (see Fig. 39). 



24. The posterior thoracic ribs do not meet the sternum, 

 and undergo a gradual diminution in size from before back- 

 wards, the sternal rib being finally reduced to a small 

 cartilaginous tip to the vertebral rib (see Fig. 39). 



25. The cervical ribs, occurring in connection with all 

 but the first three cervical vertebrae : those belonging to the 

 fourth and fifth vertebrae are short and flattened, the others 

 resemble the thoracic ribs, but do not reach the sternum. 



V. In the skull, note 



26. Its general shape : it has a somewhat pyramidal form, 

 the base of the pyramid being represented by the hinder 

 surface, the apex by the anterior extremity. 



27. Its dorsal surface is formed by a large number of 

 roofing bones, in which it is often difficult to distinguish 

 the sutures or lines of junction between the bones from 

 the depressions marking the insertions of the overlying 

 epidermic scales. 



28. The roofing bones are continued over the lateral 

 surfaces of the skull, bounding the orbits or cavities for the 

 eyes, and joining with the bones which constitute the 

 alveolar margin along which the teeth are ranged. 



