HOLOSTEI 



323 



median one may persist (Amioidei and some Teleostei). The hyoid 

 arch is formed of five cartilages : hyomandibular, interhyal, hypo- 

 hyal, and basihyal ; but the ceratohyal always ossifies in a main 

 inferior ' ceratohyal ' and a smaller superior ' epihyal.' Since, 

 however, the interhyal (stylohyal) probably corresponds to the epi- 

 branchial in the posterior arches, and should be called epihyal, it 

 is possible that the ' epihyal ' ossification below it does not represent 



[in 



FIG. 303. 



Median longitudinal section through the head of Salmo solar, L. (Modified, after Brnch.) 

 at, atrium ; 6, but bus arteriosus ; 66, basibranchial, br.m, branchiostegal membrane ; b.s, 

 branchial slit ; cr, cranial cartilage ; c.v, vertebral centrum ; d, dentary ; d.a, dorsal aorta ; 

 f.n, facial nerve ; k, kidney ; I, liver ; n.ar, neural arch ; n.sp, enlarged nerral spine ; ol.n, 

 olfactory nerve ; o.n, optic nerve ; p, pericardium ; pa, parasphenoid ; pw, septum between 

 pericardial and abdominal coelom ; s.c, anterior vertical semicircular canal ; s.o.m, superior 

 oblique muscle of eye ; sp, neural spine ; s.v, sinus venosus ; tth, urohyal ; v, ventricle, 

 valves separate its cavity from that of atrium above and bulbus in front ; va, vagus nerve ; 

 r.rto, ventral aorta ; vm, rectus muscle of eye in eye-muscle canal ; vo, vomer. 



an element found separate in the lower forms (Figs. 239, 322, 

 331). The basibranchials are usually well represented, though 

 often the two last, and sometimes others, are fused together. In 

 Lepidosteus and most Teleostei the basihyal projects forwards and 

 is known as the ' glossohyal.' In the Holostei the pharyngeal 

 teeth are usually very numerous, and may be attached to dermal 

 bony plates which overlie and become very closely connected with 

 the gill-arches, basibranchials, and basihyal. A symplectic bone is 



