300 



CLASSIFICATION OF VERTEBRATES. 



this bone may be either freely movable or firmly united by 

 suture to the adjacent bones, the first condition occurring only 

 in snakes and lizards.^ The two rami of the lower jaw are 

 usually united by ligament or by suture, but in turtles and 

 pterodactyls the two are fused. Frequently vacuities occur in 

 the jaw, and usually the component bones can be distinguished. 

 In a few dinosaurs a predentary or mento-meckelian bone occurs 

 at the symphysis of the lower jaw (Fig. 310). 



Fig. 296. Sternum and shoulder girdle of lizard, Iguana, from Huxley, d, 

 clavicle; cr, coracoid ; ecr, epicoracoid; gl, glenoid fossa; id, episternum ; mcr, 

 mesocoracoid ; msc, mesoscapula; sc, scapula; St, sternum; xst, xiphisternum. 



The hyoid and branchial arches are variously developed, but 

 at no time have they gill-supporting functions. Frequently the 

 first, or first and second, branchial arches are well developed, 

 gi\'ing rise to long cornua attached to the well-developed 

 copula. 



The pectoral girdle is developed in all reptiles — even the 

 limbless lizards — with the exception of the ophidia Scapula, 

 coracoid, and precoracoid are almost always present, the latter 

 lacking in ichthyosaurs, plesiosaurs, and dinosaurs, while in 



1 The fixed or free condition of the quadrate has heen employed in dividing the reptilia 

 into monimostylica and streptostylica. 



