70 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



being seven in the primitive sharks, a smaller number in the higher 

 groups, in which, with the loss of branchial respiration, their form 

 and functions may be altered. At first all are clearly in the head 



Fig. 69. — Diagram of early elasmobranch chondrocranium in side view, the brain 

 outlined, al, alisphenoid plate; bp, basal plate; gc, gill clefts; h, hyoid; hm, hyomandib- 

 ular; /, upper labials;//, lower labials; m, Meckel's cartilage; nc, nasal capsule; oc, 

 otic capsule; ov, occipital vertebras; ptgq, pterygoquadrate;\y/, suspensory ligaments; 

 sp, spiracle; Ir, trabeculae; v, vertebrae; I-VII, viscerararches; 1-5 branchial arches. 



region, but by the unequal growth of cranium and pharynx the gill 

 arches are carried back some distance behind the head. All of the 

 arches are cartilaginous at first. 



u 



FiG. 70.— -Ventral view of (tropibasic) cranium of Lacerta agills after Gaupp. 

 aop, antorbital plate; bpt, basipterygoid process; c, entrance to nasal conch; col, colu- 

 mella; fhy fenestra hypophyseos; Jpo, post-optic foramen; na, nasal capsule; «/, noto- 

 chord; of, optic foramen; pa, prominence of posterior ampulla; pt, pterygoid; q, articular 

 process of quadrate; tc, trabecula communis; tmg, taenia marginalis; tr, trabecula; 

 VII, XII seventh and twelfth nerves. 



The mandibular arch lies in the region of the fifth nerve, behind 

 the mouth and between it and the first visceral cleft or pocket, the 

 spiracle or Eustachian tube. The arch is divided into dorsal and 



