33° AMPHIBIA xn. 14 



modern amphibia, and are formed by the failure of the basioccipital 

 to become ossified. Paired occipital condyles have also arisen, inde- 

 pendently, in the mammal-like reptiles. 



The dermal bones covering the roof of the skull are the nasals and 

 frontoparietals, while on the floor is the large dagger bone, the 

 parasphenoid, and a small tooth-bearing vomer. The remains of the 

 cartilaginous palato-pterygo-quadrate bar can be recognized as a rod, 

 covered in front by premaxillae and maxillae, and dividing behind 

 into an otic process fixing it to the skull (autostylic) and a cartilaginous 

 quadrate region articulating with the lower jaw. This region is covered 

 by the pterygoid ventrally, the quadrato-jugal laterally, and the 

 squamosal dorsally. The palatines are membrane bones forming the 

 anterior wall of the orbit. The upper jaw is thus supported by struts 

 formed from the nasals and palatines in front and the squamosal and 

 pterygoid behind, an arrangement that gives a large mouth for res- 

 piration and eating insects, combined with the advantages of strength, 

 mobility of the lower jaw, and lightness in weight. 



The lower jaw consists of Meckel's cartilage, covered on its outer 

 surface by a dentary and on its inner by an angulo-splenial bone. 

 The anterior tips of the cartilages ossify as the mento-Meckelian bones . 



The visceral arches are well formed in the tadpole but are much 

 modified in the adult frog. In the tadpole the skeleton of the hyoid 

 arch consists of a large pair of ceratohyals attached to a basal hypo- 

 hyal. As a result of subsequent metamorphosis the ceratohyals later 

 form the long anterior cornu of the hyoid, attached to the pro-otic 

 bone. The body of the hyoid is a plate lying in the floor of the mouth 

 and formed from the hypohyal and from the hypobranchial plate at 

 the base of the remaining arches. The posterior cornua support the 

 floor of the mouth and the whole apparatus assists in respiration. The 

 sixth and seventh of the series of branchial arches give rise respec- 

 tively to the arytenoid and cricoid cartilages of the larynx. 



The lateral plate muscles of the branchial arches are well developed 

 only as the muscles of the jaws. Certain muscles of the scapula (the 

 cucullaris (p. 319) and interscapularis) are innervated from the vagus 

 and recall the sternomastoid and other muscles innervated by the 

 spinal accessory nerve in mammals. 



The muscles of the hyoid arch, innervated by the facial nerves, 

 remain mainly as the depressor mandibulae (Fig. 197) running from 

 the back to the angle of the jaw and serving to lower the floor of the 

 mouth. The jaw-closing muscles, m.m. adductor mandibulae, belong 

 to the mandibular segment and are innervated by the trigeminus. 



