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 
Fic. 61.—Diagram of early elasmobranch chondrocranium in side view, the brain out- 
lined behind. al, alisphenoid plate; bp, basal plate; ge, gill clefts; k, hyoid; hm, hyomandib- 
ular; J, upper labials; /, lower labials; nc, nasal capsule; oc, otic capsule; ov, occipital 
vertebre; ptgg, pterygoquadrate; s/, suspensory ligaments; sp, spiracle; é7, trabecule; v, 
vertebre; [-V II, 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, 
BE ee A = aop 
Fic. 62.—Ventral view of (tropibasic) cranium of Lacerta agilis after Gaupp. op, 
antorbital plate; bpt, basipterygoid process; c, entrance to nasal conch; col, columella; 
Sh, fenestra hypophyseos; fpo, post-optic foramen; na, nasal capsule; nf, notochord; of, 
optic foramen; pa, prominence of posterior ampulla; pf, pterygoid; g, articular process of 
quadrate; ¢c, trabecula communis; tmg, tenia marginalis; é, trabecula; VII, XII 
seventh and twelfth nerves. 
in amphibia two) occipital vertebrz are developed, which later fuse 
with the rest of the chondrocranium. They alternate with myotomes 
and nerves in this region as do the vertebre of the vertebral column. 
