THE FROG 57 



part of the arch ; posteriorly it articulates with the maxillary 

 arch. It is grooved on the upper and outer surface for the 

 reception of Meckel's cartilage. 



The dentary, a thin flat bone lying on the outer surface 

 of the anterior half of Meckel's cartilage. 



The mandibular arch is completed in front by two small 

 cartilage bones, the mento-meckelian bones ; these are ossi- 

 fications in Meckel's cartilage, on each side of the median 

 line. They are opposed to the premaxillae, and as previously 

 explained form part of the mechanism for closing the nares. 



Compare the mandibular arch of the frog with that of an 

 elasmobranch (e. g. } a skate or a dogfish). 



(c) The hyoid apparatus. The main body of the hyoid 

 consists of a cartilaginous plate lying in the floor of the 

 mouth. The following are its more important processes: 



1 i ) the anterior cornua, slender cartilaginous rods, one on 

 each side, extending backward and upward to their points 

 of attachment to the lower side of the auditory capsules; 



(2) the posterior cornua or thyrohyoid processes, which 

 are a pair of stout bony processes diverging from the pos- 

 terior border of the body of the hyoid; and (3) the lateral 

 processes, short cartilaginous projections just in front of 

 the thyrohyoid processes. 



Compare the hyoid apparatus of the frog with the more 

 fully developed hyoid and branchial apparatus of an elasmo- 

 branch (e. g., a skate or a dogfish). The anterior cornua 

 of the frog correspond to the hyoid arch of the elasmo- 

 branch, the lateral and posterior processes are vestiges of 

 branchial arches, while the body of the hyoid represents the 

 fused ventral ends of the hyoid and branchial arches. In 

 the tadpole stage of the frog and the toad, branchial arches 



