62 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



a pair of nasal capsules develop around the olfactory organs. These 

 are usually fenestrated and become united to the cornua, alisphenoids, 

 and ethmoid plate. In a similar way a sclera (sclerotic coat) forms 



FIG. 61. Diagram of early elasmob ranch chondrocranium in side view, the brain out- 

 lined behind, al, alisphenoid plate; bp, basal plate; gc, gill clefts; h, hyoid; km, hyomandib- 

 ular; I, upper labials; II, lower labials; nc, nasal capsule; oc, otic capsule; ov, occipital 

 vertebrae; ptgq, pterygoquadrate; si, suspensory ligansnts; sp, spiracle; tr, trabeculae; v, 

 vertebrae; 1 '-VII, visceral arches; 1-5 branchial arches. 



around each eye, but since the eye must move, this sense capsule never 

 unites with the rest of the cranium. Behind the otic capsules a vary- 

 ing number of (four in some sharks and most teleosts, in others three, 



FIG. 62. Ventral view of (tropibasic) cranium of Lacerta agilis after Gaupp. aop, 

 antorbital plate; bpt, basipterygoid process; c, entrance to nasal conch; col, columella; 

 fh, fenestra hypophyseos; fpo, post-optic foramen; wa, nasal capsule; nf, notochord; 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. 



in amphibia two) occipital vertebrae are developed, which later fuse 

 with the rest of the chondrocranium. They alternate with myotomes 

 and nerves in this region as do the vertebrae of the vertebral column. 



