HOLOSTEI 



325 



the cavum cranii, and piercing the prefrontal ; Hydrocyon, whose 

 nerves are longer and crossing the front of the orbit, has the 

 bulbus further back ; and finally, Erithrinus, with sessile bulbs, has 



pmx 



ec. 



my. 



A, Salnw sa.lur, L. ; longitudinal section through the head exposing the sense -organs, and 

 brain-cavity viewed from above (after Brnch). B, transverse section of the head of a young 

 MWA trutta, L., in the region of the fore-brain. C, D, and E, three diagrams showing the 

 development of the interorbital septum. ., auditory capsule ; al, alisphenoid ; a.n, auditory 

 nerve ; a.s.c, cavity for anterior semicircular canal ; I, optic lobe ; b.c, brain-cavity ; bs, basi- 

 sphenoid ; c.w, cranial wall ; e, eye ; r.c, ethmoid cartilage ; e;>o, epiotic ; exo, exoccipital ; /.b, 

 fore-brain ; fr, frontal ; h, hyomandibular ; 7i.c, cavity for horizontal semicircular canal ; i.s, 

 Interorbital septum ; nty, myotome ; n.c, nasal capsule ; O.H, olfactory nerve ; op.n, optic nen'e ; 

 oxp, orbitosphenoid ; p, palatine ; >>f, irefrontal ; pmx, premaxilla ; p.o, ossification of optic 

 capsule; pro, prootic ; ptf, postfrontal ; r, cranial roof; s.o, superior oblique muscle; s.r, 

 superior rectus muscle ; /;, trabecula. 



olfactory nerves passing out of the orbitosphenoid freely across the 

 orbit. The septum in the Gadiformes, on the other hand, is formed 

 below the brain-case, and the narrow brain-cavity is continued for- 



