xm 



PHYLUM CHORDATA 



79 



A similar distinction may be drawn between the primary and 

 secondary jaws. The primary upper jaw, or palato-quadrate, 

 becomes ossified by three chief replacing bones on each side, the 

 palatine (A, PAL.) in front, then the pterygoid (PTG.), and the 

 quadrate (QU.) behind, the latter furnishing the articulation for the 

 lower jaw or mandible. In the higher classes the primary upper 

 jaw does not appear as a distinct cartilaginous structure, and the 

 palatine and pterygoid are developed as investing bones. The 



Fio. 775. Diagram of three stages in the development of the pelvic fins. In A, the anterior 

 pterygiophores on the right side (Rod) have united to form a basal cartilage (Baa.) ; in B the 

 basalia (Bas) are fully formed and are uniting at * to form the pelvic girdle ; in C the pelvic 

 girdle (O) is fully constituted, and at t has segmented from the basale on the right sh'.e. 

 Cl. cloacal aperture. (From Wiedersheim's Comparative Anatomy.) 



secondary upper jaw is constituted by two pairs of investing bones, 

 the pre-maxilla (P. MX) and the maxilla (MX), which in bony skulls 

 furnish the actual anterior boundary of the mouth, the primary jaw 

 becoming altogether shut out of the gape. The proximal end of 

 the primary lower jaw ossifies to form a replacing bone, the articular 

 (ART.), by which the mandible is hinged : the rest of it remains as 

 a slender, unossified Meckel's cartilage (Mck. C), which may dis- 

 appear entirely in the adult. The secondary lower jaw is formed by 

 a variable number of investing bones, the most important of 



