THE SKULL. 



473 



cartilage becomes <]ivided into a series of segments, usually four, 

 articulated so as to be more or less mobile : and either remaining 

 cartilaginous or becoming partially or wholly ossified. Each bar (fig. 

 827) forms a somewhat curved structure, embracing the pharynx. 

 The dorsal and somewhat horizontally placed segment is known as the 

 pharyngobranchial (Ph.Br), the next two as the epibranchial {E.Br) 

 and cevatobranchial (C.Br), and the ventral segment as the hypo- 

 branchial {H.Br). There is also typically present a basal unpaired 

 segment, uniting the bars of the two sides, known as the basibranchial 

 ( B.Br). The arches often bear cartilaginous rays which support the 

 gill lamellge. 



In Teleostei dental plates are usually developed as an exoskeletal 

 covering on parts of the branchial arches. 



In the Amphibia four or three branchial arches are present in the 

 embryo. 'I'hese parts are more or less completely retained in the 

 Perennibranchiata and Caducibranchiata, but in the Myctodera and 

 Anura they become largely reduced, and entirely connected with the 

 hyoid. 



In the Anura they never reach any considerable development, and 

 are soon reduced to a plate (fig. 330) — the coalesced basihyal and 

 basi-branchial plate — the pos- 

 terior processes of which re- „ 

 present the remnants of the ^ ifPr.O / Jit 

 branchial arches. L— -.-^'— ^—^^-J^AJ 



According to Piirker the pos- 

 terior process ot" this plate in the 

 adult is a remnant of the fourth 

 branchial bar ; the next one is 

 the third brancliial bar, while the 

 anteiior lamina behind the hyoid 

 is stated by him (though this is 

 somewhat doubtful) to be a rem- 

 nant of the first two bars. 



In the Amniota, the bran- 

 chial arches become still more 

 degenerated, in correlation with 

 the total disappearance of a 

 branchial respiration at all 

 periods of life. Their remnants 

 become more or less important, 



^mx 



Fio. 330. Young Frog, with tail just 



ABSORBED ; SIDE VIEW OF SKULL. (From 



Parker. ) 



All. auditory cai^sule; in front of it is 

 the cranial side wall; A.N. external nostril; 

 (S'(. stapes; Mck. Meckelian cartilage; B.Hij. 

 basihyobrancbial plate; St.Hy. stylohyal 

 or ceratohyal; Br.l. first branchial arch. 



Bones: I'^-O. exoccipital; Pr.O. prootic; 

 Pa. parietal ; Fr. frontal ; Na. nasal ; Pmx. 

 premaxillary; ;i/.r. niaxillary; P(. pterygoid; 

 Sq. squamosal ; Qii.Ju. quadratojugal ; Art. 

 articular; D. dentary. 



parts of the hyoid bone, and 

 are solely employed in support of the tongue. Their basal portions 

 are best preserved, forming parts of the body of the hyoid. The 

 posterior (thyroid) cornua of the hyoid are remnants of the true 

 arches. Of these there are two in the Chelonia and Lacertilia, and 

 one in the Aves and Mammalia. In Aves the cornu formed from 

 the first branchial arch (fig. 331 cbr) is always larger than that of the 

 true hyoid arch [ch). 



